I believe that the 5 year old brain has the mental capacity to make a serious attempt at an age group world record for.....say.......1 mile.
A 4 year old probably does not understand what a w.r. Is and he could not make a serious attempt at setting one.
Let's apply the same question to an autistic child. Give him 1 mile on a straight painted yellow line, and I know that an autistic 5 year old could set a serious age group wr for the mile. Probably around 7:30ish
But seriously. When can someone's mentality start to dictate their physicality? As young as 4 or 5 years old??