No. The only reason I can think to lie about my age is that on occasion people who disagree with me use my age as a reason to brush off my opinion. But they'd find something other than age to try to discredit me with if they didn't know that I'm young, so what's the point anyways? Truth is better.
... At least not anymore, since I'm over 18. Not hat I really ever lied about it before. xD
(Sarcasm. I'm 18.)