If/When I do have kids i'll probably set them up accounts on the day they're born and hopefully be able to put small sums of btc into them until they're an age where I determine they are responsible enough to take over it on their own, if i believe they are mature enough when they're 12 i'll let them take control of their accounts. I would probably teach them how to use btc around the time I teach them about the value of a dollar. TLDR: when they learn the value of a dollar, but i wont give them their accounts until i feel they're mature enough.