I did say that it is unusual for a horse to take so long to run such a huge top. However the study posted by grinder shows that 6YO horses can be still run at their top level and still be within the standard deviation. The problem is not that the horse is so old. What is unusual is the size of the top and that it took so long to reach it but not incredible. Of course we're assuming that the figure is correct. If not then there may not be anything strange.

Either:
(A) The figure is correct and the horse can and did run such a new top at 6, or
(B) The figure is incorrect.

Take your choice.