Normally, most people wish someone "Ad meah ve'esrim" ([May you live] to a hundred and twenty). What do you say on a person's hundred and twentieth birthday.
I have seen the answers about why 120 connected to Moshe Rabbeinu and that this is considered the maximum age (even though Aharon lived to 123, and Miriam to (about) 133. I have also seen the references to Parshas Noach, even though there are meforshim who say the reference is to the number of years until the mabul and not the maximum age that humanity would start decreasing towards.
Note that I first heard this as a joke, but serious answers are being sought here.
The original answer that I heard was for the joke and not serious ("Have a nice day") so it would not really be appropriate for this question.