FYI, the T'au fought the Necrons and the Asuryani before they met the Imperium.
The age of the Dreadnought pilot didn't shock the T'au individual analysing him more than the fact that a species decided to confine one of its own in such a horrific way.
The bottomless hatred of the marine pilot, the fact that he begged for death, is what stuck with the T'au, not the age.
All of this was cast in an ironic light considering what happened to the T'au individual later.