When I try to explain SL to someone, or bring them in-world as a guide, the first thing I say is SL isn't a game it's a virtual world. Games have a narrative, NPCs, and friends who share the adventure's series of programed events that moves the story along. SL is a virtual world where every avatar is a real breathing, feeling person who is writing their own narrative as they go. They enter the world and ask themselves what am I going to do today? There is no winning and losing or defeating a boss level. The point of existence is you and what you choose to do with this Second Life.
Unfortunately the SL interface is complex and has a steep learning curve. it may take a while before muscle memory kicks in, and basic stuff like navigating feels natural. I've yet to discover a quick fix for this. Like learning how to drive, hours behind the wheel is necessary, but once you learn it, a whole world (literally) opens up.
There are people (residents) who log-in just to build. Some residents have virtual jobs. Some residents have talents that they share in-world. Others own businesses that sell virtual assets, and some residents log-in just to socialize. If you are new, I would explore and see what interests you. Join a group and attend group events like dances and introduce yourself. Join in on a discussion.
As for not hanging out with 18 yr olds, I get it. As a college instructor I guided hundreds of students through SL. I taught basic 3D modeling concepts, basic alpha and seamless texture making etc. Many of those students are still around. Bumping into them on the grid, outside of the classroom can be awkward (especially if they are walking around a nude beach.) I've made the joke that everyone appears 19 and athletic in SL. Real life (RL) names and ages are kind of moot. Unless they publish their age, there's no way of telling how old they are (or what gender they identify with in RL.) And it doesn't really matter. You may find an old soul who is young or an older (more mature?) resident who acts like a teenager.
After 16 years in SL, I can tell you it is worth it -- for me at least. SL has provided an escape from RL, therapy, friends, artistic challenges, love and loss. It's been rich and fulfilling. I've learned a lot about my RL self and am left grateful for the experience.