My DS had a teeny tiny bike with training wheels at age 2, it was a 10". I moved him on to a balance bike (and actually returned it 2 weeks later!) and he mastered that and has been riding a regular bike, moved from a 12 to now I think 14 or 16" since age 3. He will soon be 5. I think if LO has good coordination and balance in general, that they will get it quickly!
So fun once they can ride You can cover lots of ground much faster!
I don't know if it's the best age, but we bought DD1 her first bike for her fourth birthday. She finally figured out how to pedal on her trike last summer, so we figured her birthday would be a good time to get her a bike so she can have it this summer.
I do think it's a good idea to learn to pedal on a trike first, although some people swear by balance bikes.
We never had a trike or anything like that (with pedals, etc) for DS and then for his 3rd birthday he got an actual bike with training wheels. We took him out on it a few times and he had fun on it, but couldn't get the hang of peddling. This summer he'll be 4 (and we live in a better sidewalk-y area) so we'll take him out more often and I think he'll catch on and have more fun this summer.
If we had got him a trike or something like that when he was younger, he may have gotten a hang of the peddling a bit faster, making 3 a fine age.
DD -- 5YO
DS -- 3YO
We just got DS a 16" bike for his 4th birthday. So far, he's not thrilled. He was a late trike rider and just wasn't interested in that either. He finally learned to pedal last summer at 3.5. I don't think this new bike is hard for him per se, but he just doesn't seem to care. Hopefully when the weather is warmer he will warm up to the idea.
DS1 had a small tricycle that we kept in our house (we have hard floors) from the time he was about 1.5. By 2 he learned how to pedal, and last summer, at 2.5 we got him his first bike (with training wheels). I'm guessing this summer we'll try it with no training wheels, since Caillou does it, DS thinks he should too.
~after 34 cycles we finally got our 2nd little bundle of joy~
My IF blog
Age 4 worked out good for us---bike with training wheels.