Yeah they mean his power comes mostly from anger and anger is a dark side power so not a Jedi. He could have been the most powerful grey Jedi of all time but he will not ever be the most powerful light Jedi.
I mean that’s contextual, I didn’t have Parkinson’s symptoms at 34 but I do at 35 so for some people it quite literally is all downhill and 30 might honestly be old and that’s ignoring the homeless etc.
Sorta yes sorta no. Why it happens it’s not fully explained and as with everything in particle physics we could be super close to knowing but at the same time super duper far.