This is a situation where you will always get very different opinions and it depends on each person in the couple. Imo, I’m not a fan of equating life stages to age, particularly using age at university as a measurement. At 21, yeah, I was in uni, but I hadn’t lived on my own at that point, only ever worked PT, and couldn’t even drive so I was lacking crucial life experiences that help someone mature. If I met an 18 year old who was living on their own and working FT, but wasn’t going to uni, I would say they would be more mature than me at 21 in some ways. And speaking of uni, there are people of all ages in different classes and clubs. When I was 18, I made friends with men and women who were 3+ years older than me in class and it was never weird and we were even friends outside of class.