I'm not going to tell a girl I'm dating how old I am.
Why would you hide your age?
Because she's prettier than me. I'm feel so self-conscious about my age.
When you start dating someone, it's best to be honest as possible. Hiding your age is something that can't last forever and eventually your girlfriend will find out and become upset that you didn't tell her. Rather than risk losing a relationship, open up to your girlfriend now and be honest about your age.
I am being honest and this is my decision.
But if you are hiding your age then you aren't being honest at all. You really should just tell her to avoid future problems.