If you're fifteen, I would definitely wait a little bit! The HS Epilogues and further Post-Canon content (so, HS2) are definitely aimed at an older audience. The Epilogues themselves HAVE content warnings, written in the style of an Ao3 fanfiction, but it's a little hard to tell what's a joke content warning (as in, just a one-off Homestuck bit) or what is actually present in a serious way. I would recommend heeding all warnings as serious, just to be safe!

Out of this list, there are a few serious ones that may cause you to wait a few years before reading. Because there are no pictures, and it's just dialogue and prose/narration, it can be hard to bounce between all of the serious topics. Homestuck usually gives you a goofy break by showing you a silly panel during a serious or triggering moment, but the Epilogues don't have that. It's a more serious story on purpose, in that way. The content is similar, but it's presentation is definitely not.

If you're interested in seeing what Homestuck's Post Canon has to say (yay!), I would just be aware of your own boundaries! I'm not your Dad, so if you want to read it nobody can stop you, but just be aware it's a pretty different story, tonally, and is for the "grown up" Homestuck reader. If you wanna wait and come back to it, that's probably best! There's a ton of incredible Homestuck official stuff besides Post Canon, and a billion unofficial projects, that you can consume in the meantime!

Congrats on finishing Homestuck!! I hope you enjoyed it!!