I think one might not only have to be an NT but have to have _been_ there. For example, how do you look? Do you have gray hair, wrinkles, bifocals or in some other way _obviously_ look old enough? Then there's a thing with older women that still is around in young people's grandmothers that might make that _particular_ young person conscious of it, that they don't want to tell their age, or in any way have their age referred to, that has gotten into the culture in general, as "NEVER refer to a woman's age (especially an older woman)": she might be reacting to her grandmother, not to you.
I had an incident a few years ago while working in a church office, that made me realize that some people really do take that (not letting anyone know their age) seriously, and I'd always figured that it was just a commonly accepted humorous thing. What with driver's licenses, and being old enough to drive, and having to show the nice policeman who thought you were driving weird your license, etc, I think that has gotten a lot less since the mid-twentieth century (which is where I come from
), but it's still around a little bit. I'm assuming you're a male (johns), but you never know what somebody's thinking, for sure, or where they're coming from.
Or a middle-aged person might be (honestly) flattered at being thought not old enough to drink.