fit' can be a noun,verb or adjective.In this case,you are using it as adjective describing the suitability of this place for seniors to live.NO 'ing'.
表达老年人不适合住在这个城市 = Elderly people are not suitable to live in this city.Or This city is not fit for elderlies to live in.
the chountry where is always filled with the air pullotion that is not fit to the old prople who want to live a healthy life,for the fact that this city do not have a livable environment where has no greenery and no fresher air.
suggestion:This city has serious pollution problem.It may not be an ideal city for seniors to live in.It is especially true for those seniors wish for a healthy life style and want to live in a city with more green space and better air quality.
Comment:you are too hung up on the word 'fit'.And watch for typing mistakes.