I taught my niece how to ride her first bike @ 4 years old.
She had a little tiny little lady-bug bike with like what seemed to be 10 inch wheels.
She had her training wheels on it from about the age of 3. she rode it like that for a while and one day I just grabbed a wrench and lifted the wheels up about an inch on each side and pumped up the tires ...
after about a week i saw her riding it square on the tires without using the trainers except for turns...
so a week went by and I decided to take her outside and pull the trainers off... i gave her a few pointers and paced her down the street while holding her back a little... then i gave her a push and she was off on her own !
=)
Now i'm trying the same thing with her little brother who just turned 4... he has not had as much practice as her since he never liked her bike and he never had his own .... but he's getting there !!
So far I can get him to ride without the trainers as long as i'm helping him along a bit but he is really scared of it for some reason...
=)
He'll catch on soon enough though... he does't like to get outshined by his older sister !!!