These days I have no freaking idea what is and isn't okay.

What it really comes down to is your own personal beliefs (both of you, and any children if you have them). There are physical consequences of the age difference. My ex-MIL was 15 years younger than her now deceased husband. She's just beginning her own Golden Years, but her partner of 45 years is gone.

Of course, prostate illness usually starts striking around age 45 or so. 50% of men do develop prostate cancer. A 20 year old often doesn't have a clue about all of the physical illnesses that pop up and devastate, both young and older generations. But as you get older, the occurrence rate grows. I would be sure to have an honest conversation with your gf about these things.

If she still loves you and wants you, and you feel the same towards her ~ go for it. I wish you the best of luck!