Yes very rare and old pics and as boys they probably didn't have much choice but to wear them when told to.

They don't see it as part of their current careers and I have heard nothing about William ever being kilted when he was studying at St Andrews.

Every male Sovereign after Queen Victoria has worn the kilt and most other male royals that I think of have also.

It's something that William, at least, will have to consider at some point when he inherits the Scottish titles his father currently holds and has to take on official royal duties when it will be expected in some circumstances so I don't believe that these will be last kilted photos of him.