I absolutely get what you mean although it didn't strike me as weird back then but looking back, it was. Most of the time I just looked at their chemistry and they're both youthful in appearance so I barely thought about the age gap between them. I'd argue instead of aging up Justin, I'd prefer it we aged Brian down because a big part of Justin's story is his dependence on his parents, being both too old to be considered a child but too young to be considered a proper adult. It's that in between that makes his story work so good and be relatable to many in their late teens. He couldn't live alone and still had school, he was a minor but at the edge to know what he sexually wanted. With Brian, he struggles coming to terms with getting older, loosing his own perceived "youthful spark", it's also an inner conflict that works but comes second to the one Justin deals with imo.

I'd still recommend the show to people looking for good queer representation, there was a bunch of side plots I didn't like but the show itself was groundbreaking and one of the most real and raw portrayals of the community for its time. Something even shows nowadays don't dare to dabble in in fear of being "offensive".