My two both got them for their 2nd birthday, probably really got the hang of them about 2.5 and then got tiny proper bikes for their 3rd birthdays which they took to immediately.
If you can afford the initial outlay, get Islabikes. Not only are they super light, the resale value of them is so high, we sold ours for about £40 less than we paid for it, meaning cost per child per bike - £20 each!
My daughter was on one at 18 months. Took a couple of days of me holding the back a little bit before she got the hang of it. Then she learnt to glide and after that we had to run to keep up! Now she's super speedy (2y 5m) and I can only keep up if I'm on my own bike, especially as I'm 39 weeks pregnant! Balance bikes are great.
Balance bikes are great, easily the best toy we have ever purchased for our 2 DC. They both were on pedal bikes before they were 3 and the transition couldn't have been easier.
Both ours were on them from around 2, but I think they would have been fine earlier if their legs had been long enough. We had a Likeable Jumper which was great. I second the suggestion of an Islabike though. We have invested in the pedal bikes and they do have a very good re-sale value if you look after them.