Is it illegal to lie about age for dating young 18+ women?

No, but it likely will come back to bite you.

You expose yourself to potential liability, depending on the circumstances, and what the result of the lie entailed, such as fraud in the inducement, etc. Don't do it!

Here is my take on this, if you lie to a girl about your age and sleep with her, and she finds out, she may sue you for intentional infliction of emotional harm. As part of that cause of action, she can ask for punitive damages. See the code for punitive damages and act accordingly. Cal. Civil Code 3294 (a) In an action for the breach of an obligation not arising from contract, where it is proven by clear and convincing evidence that the defendant has been guilty of oppression, fraud, or malice, the plaintiff, in addition to the actual damages, may recover damages for the sake of example and by way of punishing the defendant. (c) As used in this section, the following definitions shall apply: . ...(3) "Fraud" means an intentional misrepresentation, deceit, or concealment of a material fact known to the defendant with the intention on the part of the defendant of thereby depriving a person of property or legal rights or otherwise causing injury.

Why do you want to do this? Put another way, what's wrong with you??

You're obviously not looking for a lasting relationship.

Honesty is a virtue. Lying to younger women is just downright creepy.