A usually female character insists on keeping her age a secret.
It's commonly accepted that no one wants to get old, and that a woman getting old is pretty much the worst thing that can happen. For this reason, a lot of people don't want their age advertised past a certain birthday, and "never ask a lady her age" is a stock etiquette rule. Someone's reasons for lying about their age or keeping it a secret may vary, from being in a business (such as acting) where they risk being put out to pasture if they're seen as too old, to vanity or a delusional wish to cling to a young person's lifestyle, to simply feeling that it's private information. Usually it's The Not Secret, sometimes even an Open Secret, but at any rate, it's bound to become a talking point, and they won't be happy about it. Twenty-nine is often the oldest "young" age someone will admit to, at least for as long as it's remotely plausible. Entering any new decade will be seen as a cause for heightened security. This character is likely to be a Birthday Hater, with jokes about them turning X for the Yth year in a row. Compare Older than They Look, Age-Inappropriate Dress, Absurdly Youthful Mother. First Gray Hair and Ma'am Shock will hit this character hard.