Part of Mark Verber's Guide to Light Weight Backpacking
One of the common questions is how young can start taking a kid backpacking. The answer is, it depends on you, the child, and your expectations. If you are expecting the child carry their own stuff and cover mileage like an experienced thru-hiker, than the answer is likely the late teens. If you have kid appropriate expectations, the answer is whatever you are comfortable taking them on a trip. Just keep in mind that what thrills a child is very different in most cases to what thrills adults. The perfect example of this was a day hike when my daughter was 2 years old. We were in the midst of a majestic redwood forest with almost ethereal lighting due to some light fog. It was spectacular. Did she notice? Nope. What was interesting to her? The pebbles under a free standing water fountain. Pick them up, look at them, toss them down, repeat. She would have been happy to do that the whole day.
It's actually not that hard to backpack with babies if you are prepared to carry some extra weight, though I did not go backpacking with my daughter when she was a baby. The biggest issue is making sure you baby doesn't get too much sun, and that you keep your baby at a comfortable temperature. The primarily limitation is typically mom (or maybe dad's) protectiveness. There's a nice write-up of one couples experience backpacking with their baby in the Pyrenees. Over the years I have seen / talked with a number of families that took their baby backpacking. Personally, I think the baby doesn't really notice the backpacking experience... the trips are more for the parents. In most case, mom carries baby in a sling or front carrier. Typically mom carries baby supplies and some or all of her personal gear. Dad plays pack mule and carries everything else. Sir Joseph showed a "Jacket Expander" at ISPO 2011 which zips into the front out a down jacket providing room for a small one to be snuggled right next to a parent. Looks like a great idea for winter trips, though I have no experience using it.
In this stage of a child's life they are growing a great deal developmentally. They are developing their walking skills. Many still put many things in their month as a way to explore: grass, dirt, small insects, toads, whatever they can catch. They have a very short attention span and get tired quickly. Taking a child this age backpacking requires a real commitment because you will need to be very patient. It's unlikely that the child will go from one place to another in anything close to a strait line. It will be from pebble, to leaf, to moss, to stick. If you are luckily it will be mostly forward in direction, sort of a zig-zag pattern. Unless the child is exception, I wouldn't expect more than a few miles in a day. When they get tired you can put them in a backpack kid's carrier. Some kids love riding in these carries, but others get restless and want out after just a short bit. Most family I have seen deal with this by the mother carrying the kids pack, and the dad carrying eveything else. This means that the parents (when junior is on board) are carrying 25-50lbs more than they would when backpacking with just the two of them. Personally, this was the age that we went car camping with day hikes because we didn't see any benefit from backpacking.
At this age kids are old enough to know not to eat the poisonous berries, stay away from the poison oak, and to blow on a whistle if they get separated from you. While most don't have the greatest attention span or focus, they are able to walk along a path for an hour or so without becoming completely bored. We have found having some sort of supplemental activity really helps kids enjoy the time. Examples of this are: walking on interpretive trails, taking lead nature walks, making it a treasure hunt (e.g. looking for interesting items you know the kids will be able to find), etc. The statement "I am tired" often means "I am bored". This becomes clear when you come across something that the child finds interested. The "tired" child will take off at a full run and happily play with energy that will make you feel tired just watching. If in decent physical shape, they are able to walk numerous miles carrying a light load. I would encourage to have children carry as much of there own gear as soon as it is practical. We found our girls wanted to carry a pack just like the parents. Get them used to carrying something as early as possible. When my daughter was 4 she has a small Eagle Creek backpack which had a built in whistle. My daughter carried her own water, a snack, plastic plant & bird identification cards, a nature notebook & crayons which she draws pictures of interesting discoveries, and a toy (stuffed animal). By the time she was 7 she was also carrying most of her clothing and a sit/sleep mat. I would recommend keeping their carry weight less than 10% of their body weight. I have also found that while there are some children who really enjoy hiking, most don't. There was a good article about backpacking with young children. My daughters answers at that age (5) would have been almost identical to Chase. At this age I wouldn't plan to cover more than 3-4 miles in a day, though there are some exceptional kids that will do more distance than this.
At this age one of the best things to do is bring a friend or three. Kids at this age really enjoy being with their peers. If they are will friends, a lot less effort has to be put into keeping them entertained. It's also at this age that some children are able to take a sense of accomplishment from the act of hiking and can really enjoy it. That's not to say a younger child can't, just that it's unusually. At this age I still recommend keeping their load to around 10% of their body weight. With reasonably light weight gear it's possible for kids to carry all their gear at this age, though it might be necessary for a parent to carry one of the more bulky items (like the sleeping bag) because their pack doesn't hold enough volume for all their items. While dedicated outdoor parents might take children backpacking before this age, it seems that this is the age that many kids go on their first trip. Unless the child is already an avid hiker, I recommend keeping trips below 4 miles where the destination (and ideally the hike) have kid friendly attractions. There have been exceptional kids that have thru-hiked with their parents, but most are unlikely to do more than 6-8 miles in a day.
By the thing a child is in high school, they are able to carry all their own gear. The amount of shared items I would have them carry would depend on the child: level of physical fitness, if this is the first time they have gone backpacking, the type of backpack they are using, and the degree to which they want to pitch is with the shared items. Distance and difficult are limited by their physical fitness and interest level. Boys tend to eat a lot during high school because they are growing at lot.
It's a bit harder to find good quality backpacking gear for kids that it is for adults. The first issue is finding nicely made items. That's not to say kids items aren't availible. There are dedicated kids companies like Mountain Spouts, Mole Hill Mountain, and most main line companies have kids lines. It's just that there are typically a lot more options for adults. The next issue is cost when you know that within a year or two the item will likely be outgrown. Ideally you can hook up with someone who has kids a bit older than yours and arrange for hand-me-downs. I would also encourage you to plan to pass on items which are of good quality to someone once your kids have outgrown them. We didn't have anyone to get hand-me-downs from. Thrift stores and used kid clothing stores are often recommended. In the SF Bay area I typically didn't find good kid's outdoor clothing. When we were in Tahoe I found a fair bit, which makes me think that where you are has a strong influence on what may be in the thrift stor
If you are purchasing new, I would suggest checking out places that specialize in high discounts, or outdoors companies that have regular clearance sales. We found the most of our kids clothing at sierra trading post, patagonia specials, campmor or REI clearance sales, and a few items from land's end. While it might be surprising, I strongly recommend looking at Patagonia kids clothing. While they are expensive retail, they are almost reasonably priced on sale, at the outlet, or best at an outlet during their memorial day / labor day sales. We have found that Patagonia kids clothing not only to be good quality, but seems to be cut in such a way that it takes longer to outgrow them. In a time that my daughter seemed to outgrow jackets every year or so, her Patagonia jacket lasted 3 years before she outgrew it, and it was still in good enough shape to pass it on to another girl. If there is something you need right now, and can't find anywhere, you might check out kidssource.com which focuses on outdoor clothing and gear for kids.
Rather than trying to find a good deal on an item as it was needed, we would often purchase something in a larger size in advance if we found a great deal.. This resulted in a number of great deals that were used in future seasons. Example of this include a kids DriClim windshirt for $11, and Patagonia Puff Vest for $20, a kid's size external frame pack for $10.
My packs page has a section on kid's packs
Typically boy scout backpacking is the opposite of ultra-light backpacking. Everything including the kitchen sink (or at least a dutch oven to make cobber) is taken. Gear is typically extremely heavy-duty to survive the riggers of younger boys. Some good references for scouts include: