On a non-ps 757 all the "inner"/middle armrests can be raised except for those in the exit rows and speaking of exit rows, exit row seats are a bit narrower than non-exit row seats as the tray table is in the non-raisable armrest. Also, you say you were in 21F with "a kid" next to you-21F is an exit row and I'm wondering how old the kid was as iirc, the minimum age is 15.

Also, lots of good info on 757 seating over here: http://www.flyertalk.com/forum/unite...estic-p-s.html