People should not be forced to retire, and they should be allowed to continue working as they feel appropriate.
I want to say as they feel it is OK. Is this is a correct usage of the word?
It would be better to say "...feel is appropriate".
When you say "They feel..." the adjective is usually one that gives an emotion. For example "they feel happy". It is not the people who feel appropriate. The people feel that the time for working is appropriate. So the shortened form would be
People should work for as long as they feel is appropriate.