If she asks, make her guess and add a couple of years to whatever she says. But don't go up a decade (eg. if she says 38, don't go into the 40s).
If she doesn't ask, there isn't a reason to mention it.
Edit: letting her know your real age is also an option if it roughly corresponds to your appearance. In my case, they always guess around 10 years younger than my actual age. I usually go for women 15+ years younger than myself and find I get better results letting them choose my age.
Eastern Slavic women aren't bothered by age differences, but others tend to be at some level.
If she doesn't ask, there isn't a reason to mention it.
Edit: letting her know your real age is also an option if it roughly corresponds to your appearance. In my case, they always guess around 10 years younger than my actual age. I usually go for women 15+ years younger than myself and find I get better results letting them choose my age.
Eastern Slavic women aren't bothered by age differences, but others tend to be at some level.