Is the opposite of 上床 下床 or 起床?
Alright, to me, 起床 means the end of sleep and getting up. 下床 means the opposite of being on the bed, as in your example, two elderly people 从来不下床 may mean that they have health problems so they have to be lying on the bed, but they may not be asleep when they are on the bed.

上床 can be used to refer to the action of going to bed, like 我一般晚上 11 点钟上床。(I usually go to bed at 11 pm.) Also 上床 is the common saying of having sex.