The Christmas present that started me off was a crystal radio kit. This was the mid-forties, when I was about five years old. Got it in the evening, put it together in time to wake my parents the next day with the weather report.
In subsequent years there were Lincoln Logs, Lego, an Erector set, chemistry set, and a lot of model airplanes. As somebody earlier mentioned, many flew like scalded cats when you put a Jetex engine on them. Others didn't survive. That was about when I started making model rockets. Making them, because there weren't any kits back then. I survived.