I had the sort you couls tie onto your shoes when I was 5, and could skate with them after a few goes.
DD1 I bought skates (disco roller boots) for her 6th birthday but she couldn't get the hang of it for another year really.
DD2 I took to a roller disco with DD1 at just turned 4. I expected to be dragging her around but she started to get the hang of it after about ten minutes, then soon after was skating around on her own quite confidently.
I guess it depends on the child.