Y'all can correct me if I'm wrong, but I think that they're both at least 18.
Considering that they went to a love hotel, looked for an apartment together and were even given a key to watch adult movies, they can't possibly be underage.
Of course, the uniform may imply that she's a highschooler, but there certainly can be highschoolers who are also 18, and there are colleges with uniforms, so I feel like it's not a good indicator.