Hi there J.,
My son's been riding a tricycle/big wheel/scooter since he tuned 2 and i had him try a cousin's bike with training wheels recently to see if he was ready for a 'real bike' (he's almost 3 1/2 now). Seeing that he loved it, my husband and I just ordered the 10" wheel Radio Flyer bike with training wheels yesterday (it has great reviews).
Check it out at: http://store.babycenter.com/product/classic+red+10%22+bic...
(it's on sale right now also) I love that he'll be able to grow with it - when he gets more confident I'll take the training wheels off!
I think every child is different, but as long as he/she can handle the physicality of a bike, and is willing to bide by your rules (ie. not to ride without a parent nearby, stay on the driveway/sidewalk only, etc.) I think a bike at this age is an inexpensive and surefire way to keep our kids active.
Best of luck, hope this was a little help.