For the oldies, senior's card gets you a discount!
I am not a Senior - 60 and above - but you are correct, although it doesn't appear there's a Discovery Pass for seniors (but as Mattg indicated, there is one for children). Seniors pay the concession rail fare but must pay the $88 single - one way - sleeping berth fee as well.
NSWTrainLink won't book strangers of the opposite sex in a sleeping cabin together (to clarify: girlfriend and boyfriend, or friends, fine, but if it's me and Miss Jane Bloggs, no). So sometimes there may be a spare upper berth if there's not been any bookings from a person of the same gender.
You can also encounter someone say getting in the sleeper at Wagga Wagga for the trip to Sydney, or Goulburn/Yass Jct for the trip to Melbourne. In reverse, they can also leave then, but most doing it are quiet. You won't, however, be woken up at 0200 or 0300 by someone alighting in the middle of the trip at say Cootamundra as one can't book a sleeper to or from there.
There is some sort of choice with brekky: it can be a bit more extensive than Mattg implied, although for those of us who don't often eat at 0620 hours we may not want much.
Try to obtain a lower berth as it's easier to get in and out of, although the ladder to the upper berth is stable and well designed.