Yes, it's definitely a red flag. My daughter was dating a guy that lied about his age two different times until she found his driver's license. She started looking into things and found he had been lying about being in school, too. He even had her drive him to school on days his car was broke down. She found he had never enrolled in school.

I would be very concerned if someone lied about something so simple as their age, when dating someone. Age isn't the important thing the lying is.