The only time I felt I needed to lie about my age was when I was getting into a fresher (1st year uni student) when I was in my 2nd year. Said it was my 19th birthday, when it was actually my 20th!

Basically age corresponds to certain social groups and classes, that depends whether you're at university or just out in town etc... If you're out on the town then anything goes if you feel it will help you 'fit in' with that group and get you laid, then go for it.