They say you must be 16 to drive, 18 to vote, 21 to buy alcohol (most places), and 50 join AARP. Maybe you're just not feeling your youthful "age" anymore. Been acting immature lately? Don't have an ID that matches your "intentions"?
But did you notice? When asked that question - were you asked WHERE your age should be measured? Yeah, they'll insist that they meant, "here on EARTH", but if they didn't ... You can now give an honest and informed answer* - right down to the Years, Months and Days, based upon your age on a convenient planet right here in our solar system! Choose any of several appropriately confusing responses!
Simply provide a birth date - and run the calculation. Enjoy!