I was in the pub Friday night and banteringly asked the delightful Irish girl working the bar how old she thought I was. (I like to think I appear more youthful than my age).
Well, her answer was one year more than my actual age. So I called her a name - the one that rhymes with 'witch'. (It's OK, we kid around all the time).
She then demands of me, "And how old do you think I look?!" I immediately replied, "Twenty-six!".
A cocksure grin came across her face and she told me, "Gooood answer!".
So, I figure twenty-six is a good number to offer up when speaking of a women's age.
Claiming to be younger than ones years always seemed to me to be counterproductive. That said, I would make it '24.'
Twenty-five is the watershed. Things start going downhill physically after 25. ;)