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 Maam Shock will hit this character hard.

Examples:

• This is a Running Gag with Jenna from Thirty Rock. At one point she survives a rapid-fire interrogation from Jack by rattling off what are clearly memorized answers to questions such as what her high school prom theme was. Also, in one episode Liz pretends to be in her twenties to justify a relationship with an allegedly twenty-five-year-old guy who turns out to be only twenty, and she agonizes over the fact that, for her usual feminist reasons, she's always promised herself she would never lie about her age.
• Auntie Mame. It's a big deal when she begins "admitting" she's forty now and it's time to take on a more mature role, which doesn't happen until her fifties.
• In Are You There God Its Me Margaret, Nancy volunteers this information about her mother.
• Joan on Mad Men seems to have done this, as somebody in the office plays a prank on her by pinning a photocopy of her driver's license on the bulletin board with her date of birth circled.