One has just become an adult and is still a teenager btw, and one is past the mid-twenties point, huge imbalance. 18 years old are kids, my baby brother is 18 and I'm 23, I can never imagine seeing his friends as anything other than immature kids.
I think the age difference will be seen as a problem later when Tamura finds out. Right now, she doesn't even know her age. And she doesn't seem surprised that she hasn't seen any men yet, so I'd like to know why.