I find it funny, I started my re-read of Pride and Prejudice this week, and the main character, Lizzy Bennet and her closest friend Charlotte Lucas have this exact age gap. Lizzy is 20 and Charlotte is 27. I don't find anything odd in how their friendship is portrayed, and while of course it is fiction, Jane Austen was a realist.
In the real world it's also not odd and unless the 20 year old is extremely immature for their age, it's not a massive maturity gap either.
Interested to hear what prompted this thread. Does your 20 year old have older friends who are bad influences, or something along those lines?