DD - 5th birthday (had a balance bike for a year before that so never had stabilisers)
DS - 3 years and can use DD's (16") pedal bike, but its really too big for him and hard to control...so he will have to stick to the balance bike until his legs grow a bit and he gets his 16" for Christmas.
Sounds like there is a huge range and it depends a lot on inclination and interest and how much time and space they have to practice...BUT, I am really impressed with how much balance my two have learned from having the Balance Bike, and I am thrilled we skipped the stabilisers stage, as I think DD would have clung to the security of them and been very hesitant to ever have them off as she is naturally quite a cautious child.