My husband is 18 years older than me, we are mentally on the same wavelength!
When I was 19, I went out with someone for six years who was 23 years older than me, as woman saw it, or I was 23 years younger than him, as blokes saw it! I've been out with people the same age as me, and we have had a great time. So age to me is not relevant. The noticeable differences when I was 19 were:
I'd spend all night playing computer games, he spent all day doing the Times Cryptic crossword. I taught him to play, he taught me to solve cryptic clues; I was happy watching teen films, he was happy watching David Attenborough, no real compromise was made there!; I liked clubs, he liked restaurants, so we went to pubs instead. Any relationship is about compromise.
That relationship, as stated, only lasted 5 years. With the age difference between me and OH, people presume I am older than I look, and he is younger than he looks! I know 18 year olds more mentally mature than my husband (but I wouldn't change him for the world)