We got our son a likeabike Jumper for his third birthday a few weeks ago (metal frame not wooden) and he just LOVES it! They are not cheap and I dont think I'd have been comfortable with him having one any earlier as he zips about on it - he got the hang of balancing very fast and now loves going down hills on the common!
My only concern is that it has no brake - they are supposed to use their feet. On a recent holiday my son picked up a bit too much speed and then was scared to stop, he baled out and got quite badly scraped - I shudder to think what could've happened if he had been near a road. For this reason I'm quite selective about where he rides it and he always wears a helmet. Having a scooter first has definitely helped him build up some road sense but essentially I think this type of bike is best used on the commons.
(BTW.. Likeabike do sell a brake separately that we are considering buying.. plus check out this video on youtube if you want to see what the bikes are capable of... enough to turn a mum grey!!
http://www.youtube.com/watch?v=Bxp4TjAkSL8