My 3 main recommendations were already mentioned but I’ll mention them again for visibility.

Stormlight Archives by Brandon Sanderson. There are actually multiple main characters, but the world building is top notch. I haven’t finished the series yet (it’s my “read during lunch” series atm) but so far there are 2 male main characters in the 18-25ish range

The Codex Alera by Jim Butcher. The main character starts around 12 and there are arguably multiple main characters, but it’s a damn good series. Think ancient Rome but everyone controls elementals

Lightbringer by Brent Weeks. A fantastic series where the government and religions of the world are built around the magic system. The series happens over a few years but the main character starts at around 15

Since I mentioned Lightbringer I also want to mention The Night Angel Trilogy by Brent Weeks. It’s a dark fantasy series featuring magically gifted assassins. Most of the good guys are criminals and murderers. It’s a brutal series with an excellent cast of characters and a world that feels like it’s been through hell. The main character starts off as a kid, but is in his 20s by the end of the series