People telling white lies about their age is nothing new. Most people do it at some point, and many of us have been telling others a false age for years.
But if you give a false age or date of birth to the cops, you could be in real trouble.
Penal Code 148.9 makes it a crime to give a false identity or false identifying information to the police.
Penal Code 148.9 is most commonly prosecuted in the context of people giving a false or fictitious name to the police – especially during a traffic stop. But courts have ruled that this section applies to any identifying information, including one's date of birth.
We see this come up is situations where a driver has outstanding warrants, or is an undocumented immigrant who can't get a driver's license. Upon getting pulled over, the driver will often try to avoid trouble by offering a false identity.
There is some uncertainty as to whether the Fifth Amendment allows one to remain silent in situations where providing his identity to the authorities would incriminate himself. We won't try to address that question here.
Suffice it to say that if you get caught pretending to be someone else to the cops, you could get yourself into double trouble.