I started backpacking/hiking with my daughter when she was 5 (now 6). My son, who I didn't even know was interested in hiking with me started when he was 12 (now 13).
I started doing local dayhikes ranging from 2 to 3 miles then slowly increased the mileage. My son can now handle 8 to 10 miles per day and my daughter can now handle 5 to 6 miles per day.
One thing to note, though their legs are strong, they still require lots of breaks and snacks. I always ask them if they're ok, if they want to stop for a break, or even if they want to go home already - so far I've been lucky, the answers so far have always been "no Dad, we're good, let's just go so we can set up camp".
When they do join me on trips, I just make sure that the car isn't too far to get back to just in case and also I pick trails that are not so desolate.