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.
I was in a similar boat and became close friends with a coworker in his 50s when I was in my early 20s. He became a mentor of sorts who helped to guide me through a lot of young adult issues, and I was thankful for him. I’d encourage you to be open minded about your son’s friendship unless you are sensing red flags from his friend.