I have a Senior ticket. While traveling on amtrak, am I prohibited from sitting in an unoccupied seat reserved for passengers with disabilities?
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 they are not likely to ask a person old enough to travel on a senior ticket to move on.
I am not familiar enough with Amtrak to tell you whether you can use the seat if you are not handicapped.
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-deckers, people who have trouble with stairs will buy a ticket for the lower level. I've seen the conductor on a crowded train ask people to move upstairs if possible. Even if you have ticket not specifying a lower-level seat, though, I don't believe there would be an issue if you are on a Senior ticket. Most passengers prefer the upper level unless they have a reason to stay below.