My father is 20 years older than my step-mother and 7 years older than my mother (and just as a point of clarification it was not the age issue that led to the divorce, it was something significantly more complicated.)
My sister-in-law currently has a boyfriend who is 25 years older than her (though I personally think that is him having a mid-life crisis.)
I think the thing that is getting to you is that you probably consider yourself an "adult" while someone who is 20 is still a bit of a youngster (like you said, younger than your cousin.) The thing to remember is that it isn't your cousin [img]http://www.**********.com/iB_html/non-cgi/emoticons/new/smile.gif[/img] Just go out and have fun, if you get along then great and don't sweat the age issue.
And if you don't hit it off then you can blame the age difference and make a clean get away (j/k [img]http://www.**********.com/iB_html/non-cgi/emoticons/new/smile_n_32.gif[/img] )