There is an age when starting hike with your kids? Yes: zero!
You can bring you child with you from when they are born. Probably better to not go above 2000 meters when they are just born but I know people going at 3000 meters with their kids. And nothing wrong happened. The most difficult age to go with kids is between 2 and 4 when their weight is a lot but they probably will not be able to follow you in the hikes around the Alps and the world. After that, at 5, if they want they will be able to follow you wherever you want. This was the case of this wonderful Danish family with whom we spend 2 days in the Italian Alps. We sleep in the shelter and saw very beautiful alpine environment with two kids of 5 and 8 years old. It was their first experience but as they were motivated there was no problem at all. Actually we reached even a pass at more than 2000 meters. They really did a great job! And the night in the shelter was also a nice experience. Tracking stars and waking up in a natural environment was great. It was a real pleasure for me to guide you guys! The kids were enthusiastic and it was refreshing for me to spend time with them. We played at the river as when I was a kid and I must admit it have been almost as funny as when I was their age. Sometimes we forget about our child that lives inside us. But he is there waiting to be fed. That was a very great opportunity for me t0 become a child for a while in this fabulous Trekking with children in the Italian Alps.