The above sign would be improved by the insertion of a definite article before elderly:
This restroom is for people with disabilities and the elderly.
This would make it clear that the restroom is for (i) people with disabilities, and (ii) the elderly. The definite article would help mark the start of a new, separate noun phrase, so that elderly would not be construed as being part of the postmodifier of the head noun people. Of course, we might also choose to repeat the preposition (i.e. for the elderly), but perhaps an even clearer way of wording the notice would be:
This restroom is for the elderly and people with disabilities.