My wife & I have an 18-year-old son. He started university in September, studying medicine. There he met this 41-year-old man - a classmate, and they became really good friends. This man has just started university now at this age because apparently he was born poor and in the first half of his life was focused on making money, but medicine is a dream of his. Son has always been an introverted, shy, socially awkward kid with little friends, but now goes out often. Honestly my wife & I are uncomfortable, and we can’t help but side eye the dude.
Depends entirely on the person. I have friends that are a similar age gap to the one you mentioned, it just never occurred to me to think of it as weird.
We’re all into the same hobby and that’s how we met, but we also like similar video games, music, TV, etc, so there’s plenty to talk about and have a friendship around. I don’t think about their age really at all, it just doesn’t come up.
But yeah, it entirely depends on the individual, on both ends. You can have a weirdo older person, you can have a weirdo 18 year old. Just got to weigh up a person and try to figure out what their deal is :-)