We were never told actually. I think we just assumed that because she was studying. I thought she was a high-schooler. But then we saw a very old woman also being a student in her school (college?) some chapters ago. So it seems its one of those public places where anyone can enter regardless their age.
Her school is a "correspondence" school for older (or other troubled) people trying to get a HS diploma or its equivalent (like a GED in the US, I guess).
In chapter 2 we are told that she is 21 years old (I mistranslated it to 22). Besides being the protagonist's older sister It means that she is older than Him.
What do you want a 22-year-old to look like? With gray hair and wrinkles?
So many people on MangaDex have no idea what a 20-29 year old looks like. It's probably because the 14-17 year old teenagers are usually drawn like they're in their 20s.