Unreserved Northeast Corridor trains have a few wheelchair-friendly seats, which can (unlike regular seats) be reserved specially. As the number of tickets sold never exceeds the number of seats, if someone has a special reservation you will always be able to find another seat.
On the double-decker Capitol Corridor trains, and I assume on other double-...
Do you need seating for handicapped people?
If yes, you can use it.
If not, please do not use it if there are other seats you can use.
Age is not the deciding thing here.
A 20 year old girl or guy who looks healthy might have an invisible handicap and need it much more than a healthy 70 year old who can walk on to a seat a bit further into the train. But ...