I'm pretty ashamed about the age of my girlfriend.
What makes you so ashamed about that sort of thing? Is she too young or too old?
I'm just getting very tired of this phase in my life and maybe it is time for a change.
You should consider breaking up with them then, or at least letting them know your feelings. You should never feel ashamed of someone that you love, and if this is how you feel then it is better for both of you to let them know about it now in any way that you can. Otherwise you could be in a terribly unhappy relationship for many years.
You're right, it's a good time to start letting go. She will find another guy who has a job and a home.
It's very mature of you to realize and accept that this situation is not working for you and to cut things off before they get any more serious.  The both of you will be better off in the long run, I promise!