Thanks for this great question. I think your cousin is fortunate to have such a caring person as you. My answer is based on my personal experience as an 18 year old college freshman at summer school on the east coast where I joined the summer chorus. There I met an incredible woman graduate student 10 years older named Rachel. I liked her from the first moment - she was so animated, smart, cute, flirtatious, had a beautiful voice and was such fun. We started having great conversations after evening practice. One evening she asked me to walk with her back to her apartment, invited me to come in and have a beer. We listened to some music, talked for hours and ended up kissing a lot and then some - all at her initiative (I had wanted to almost from the beginning but didn't dare make the first move.) After that, we were together every night. It blew me away that she liked little old me that much. She always treated me as an equal, even deferred to me occasionally in some things. Unfortunately, our romance tailed off after she went back to grad school at Berkeley. This experience helped me greatly to gain confidence in myself. Ever after, I always felt comfortable with "older" women and even had a couple of other such romances during my younger years. What sweet memories telling this story brings back!
The guy your cousin likes is 21. That's 3 years older than I was. He has probably completed his education, is working or also going to grad school. I'm sure, he is far more mature and self-assured than I was at the time of my summer romance. My suggestion is to leave it alone. If he proves to be immature, your cousin will soon realize it, surely be dissatisfied, and she'll end the relationship.
