My 5 year old twin boys have had them for over year and they love them. My DH bought them without me knowing, and I was a little ticked that he did that. First of all the smallest size he could get was a size 1 and our boys were in a size 12 at the time, so the Heelys were very very big for them. My second thought was what they heck are you buying 4 year olds Heelys its just an accident waiting to happen.
Well I was totally wrong, they both put them on and just starting rolling around the house in them. They practiced and practiced for a couple ofweeks in the house and they have been rolling around ever since. Its amazing to me how coordinated they are. I shouldn't have been surprised, cuz they have been riding two wheeler bikes since they were 3 1/2.
They have had a few falls, nothing major. But one time my one son had them on in our local grocery store and he took a really hard fall on his butt. But he got right back up and starting wheeling again.