I'm with HK...give either one or the other, age or grade level, not both.
For some reason we almost always get asked both questions (one as a follow-up), ala:
"Sixteen... so you'll be a junior this year?"
-or-
"You just graduated, so you're eighteen?"
Our official tactic with DD has always been: don't announce it, but you don't have hide your accelerations either. If people would only ask one of the two questions, we'd have avoided many an awkward conversation/commentary. But, typically, we get one question, answer it, get the next, pause...awkward smile...pause, then answer it while bracing for the judgement.
Sometimes the age/grade outing comes at us sideways, like the following, which is more amusing than annoying because of the circuitous route that brought us unexpectedly to the reveal. DD16 was talking with the father of a fencer in her foil class. Many Canadian universities have fencing teams, so the discussion of DD's college search touched on Canada. DD said we briefly looked into some of the fencing schools up there, but decided the custodian rules would be a hassle.
"What custodian rules?"
"Um, the ones that say I would have needed to arrange for an official custodian, or have one of my parents move to Canada, since I'm still 16..."
Best of luck,
-S.F.