Finnmark: I reckon I was about 4 - certainly before I started school. In fact, I can't remember when I didn't! Sounds a bit precocious, but there we are. I seemed to discover the 'technique' quite naturally - nobody ever showed me. I would play with it in bed, it would get hard, and then there'd be this lovely feeling like a bottle of fizzy drink exploding - which was how I thought of it.

I guess we've all got our memories about the point in life when this fascinating 'hobby' began, and there can't be a "too young" stage. I don't think this was what the questioner really meant to ask ...