Mic and Jerrianne Lowther thru-hiked the AT with their daughter Kyra who was 10. Mic wrote Walking North published in hardcover 1990 (only 34 copies!) and in paperback 2001, Elton-Wolf Publishing of Seattle. Originally from Minnesota he now (2001) 'teaches fire safety to children all over Alaska."
I do not think it makes any difference at what age you start backpacking with your children. They will either like it or not. My parents did not start me, summer camp did. My own kids started at 10 and under, I have introduced teenagers and younger to backpacking by organizing weekend hikes from school. I got other teachers to join me. Sometimes we just had 4, other times 8-10. We started with day trips, camping at a base. Take them anywhere near water, pond or stream and you will not have to think up anything to entertain them. We also had touch football games, or ultimate frisebee games at the end of our hikes. Canoe trips are fun, too. Remember, the camping is probably more fun for them than the hiking. My favorite quote overheard when answering a parent’s question about the trip: “We had a great time except for all that paddling.” My three kids are in their twenties and they will still hike with me. One daughter is joining me for an end-to-end of the JMT next summer. She taught me about tarp camping. My son works for a nationally know company that sells outdoor gear over the internet. On a good “powder day”, (Utah), his boss makes them all take a day off to go skiing. The first time he reached a summit, he was in my backpack! In short, start them whenever you can. My kids still remember some of their earliest hikes.
rambler