How a boat was cared for has a lot more weight than just age alone. Yes it's true that as they age there are things that can go wrong, and even possibly something that the owner doesn't know about or can't control. If a boat was cared for, it'll last a lot longer than one that wasn't. As a great example, up until last year I owned a 1986 Ranger. It was in better shape and had less issues than multiple boats in our club that were from the 2000's. I had my fair share of projects with it, but those were projects I chose to do like adding compartment lights. The only thing needed to be replaced were the aerator and bilge pump which were easy fixes. From my experiences the older boats that were cared for are pretty easily spotted. If you can find an older boat and it's in great shape, I'd rather buy that than a newer one that's been beaten to death.
As far as aluminum vs fiberglass that's a personal decision. Aluminum is more forgiving if you have no clue what you're doing, If you've owned a boat before and can get it to the dock without smashing into it, or you don't fish shallow rock infested rivers, I'd go fiberglass, but that's just me.