All your examples can be correct since 年を取っている is not wrong. (It sounds to me you are old people, though.) At the same time, all those expressions, including mine, can be wrong. It totally depends on the clause(s) that follows it. My examples are for the structure 今はもう若くありませんから、若いころは~でしたが、…….
Wait! I now realize that you wrote "the title" in your initial post!! Then, I would use もう若くない. Sorry!!