I'm pretty much just echoing what others have said, but if you've started to notice a wee difference yourself then that's probably the best indication you can get.
I didn't see any real change in Jorja's ability on the hills until she was about 11 or so, and even then she could still go all day, but it was the slower "recovery" over the next day or so that was the big thing - she was like myself(!), kind of stiff and sore, which had never previously been a thing.
I decided to consciously cut down on the miles thereafter (6 or 7 seemed to be the sweet spot) and she has kept going. We got to the 50% Munro mark at the end of last October, and she turned 13 today, as it happens.
I also found that the terrain made a huge difference to her both on the day, and afterwards. Grass/bog/softish paths were much preferable to consistently rocky stuff.
Hopefully Hughie's got a wee while yet on the hills!