The context of which I used is perfectly fine. I was reffering to a guy who has been working for 20 years, and want to change his carrer path. So, I would like to point out that I know this sentence is not independant. Maybe next time I have to be more explicit.

I like Jonathan's comment

The sentence is grammatically correct but not very clear in meaning. If you're trying to say that you're not sure but are guessing the person's age or stage in life, I would suggest: "Shouldn't you be about the age to retire now?" OR "Shouldn't you be approaching retirement around this time?" OR "I'm guessing you're at a retirement age." Very importantly, take note of punctuations and put a comma at the end of the sentence if it will be written as a question.

It's grammatically correct, but nonsensical. If I'm interpreting your intended meaning correctly, I'd suggest: "Since you've already reached retirement age, why haven't you gone ahead and retired yet?"

It is correct, however it would also depend on the context in which you are asking this question.