It's not odd, even less odd if the 2 have shared interests and are similar in maturity levels.
My step sister is about 7 years younger than me and when we met the gap was huge, she was a young child, I was a teenager with very different interests. Fast forward decades later I'm 36, she's 30 and the gap is nothing, especially because we are both parents to similar aged kids so we are at the same stage of life.
One of my friends is a year younger than my sister, again a parent, settled down so we are also at the same stage of life.
If someone 7 years younger was horrendously immature then no I can't imagine being friends with them and the same applied when I was 27.
Personally I would apply the same rules to relationships as I would to friendships which for me is 10 years either side but as I get older I could see how it could easily go to 15+ there is only so much maturing a person can do.