You know what, as cunning and deceitful as it is, the "21" response that the guys seem to be giving is a good one! If someone said that to me, I would commend them for being close--I wouldn't even THINK that it might be a standard line! Guys are trickier than I thought! I need to watch out! Just be careful not to use this line with a woman who is quite obviously older--if she has a sense of humor she might laugh and tell you, but if she's touchy about the age thing, you might end up on the sharp end of a long stick!
*warning, warning!!* Don't try this line unless you're a charmer =P