"senior" doesn't really work as a translation of "Senior", because it begs the question 'senior what?'. It needs to be followed by something else.
I hear "senior" on its own fairly frequently in American English. However, in British English we'd tend to use "senior citizen" or "elderly person" if we're being formal, or more colloquially we might say "OAP" (Old Age Pensioner) or simply "pensioner".
der Senior = the elderly person
der Älterer = the elderly person (which is probably used more often)
You can't really use "Älterer" on its own. You could say "ein älterer Herr" or " eine ältere Dame".
IMHO, leo is better than dict.cc but it is also community edited. I suggest to go with pons. Generally, I'm a big fan of crowd work but compiling a dictionary really requires expertise. There are so many subtleties to consider (which meanings are distinct, which words are colloquial, slang or socialects, which expressions are used only in certain regions, what differences are there between dialects, which usages are outdated, and so on and on...) and you have to work very systematically and consistently - laymen simply can't do this. It's really a different task than - say - writing articles for Wikipedia since you have to have a good and complete understanding of the whole language.
'Älterer' is a nominalized adjective and follows the declension rules for attributive adjectives. So, it's "ein Älterer" but "der Ältere". Christian already pointed out that the word isn't used very much in singular.
I said "old person" as well and it was not accepted, even though "old people" was accepted for "die Senioren."
In British English "The senior works" doesn't make any sense. We talk about a senior citizen / pensioner / old (or elderly) person (or man). "The pensioner works" should also be accepted, i believe.
In german is there a distinction between Senior and Rentner?
Would "Der Rentner arbeitet." would have a different meaning?