We started ours on a balance bike when they were two. It took them a while to get the hang of it - 2.5 in the first cast and just over three in the second. But, once they had got the hang of it they were able to scoot around at great speed and really enjoyed it. Our eldest is now on a pedal bike, which he switched to at 4. It took him a little while to get the hang of pedalling (especially starting from stationary), but otherwise took no real teaching to ride it as the balance bike skills were very transferable. We could also go straight to riding on the (very quiet) roads round us as his bike handling was generally very good.
The main thing is to get one that is light enough for them to be able to easily handle (e.g. pick up and put down) otherwise they won't bother with it.