which one is correct?

Since there is no article, it should be senior citizens. That is not a sentence, it is a sentence fragment, so it's a little difficult to know just what you want. If it read "the income of a senior citizen" or "the income of the senior citizzen," that is, if there were an indefinite or a definite article, then you would use the singular. As it stands right now, you want the plural.

Your second bu
is redundant.

The second is correct because the first makes no sense.

This could work, if a name was added,

The income of senior citizen Smith

Bollocks