I've played guitar since I was 12 (I'm much older than 12 now), but I've learned other instruments in adulthood (drums and bass, mostly), and I've taught guitar lessons to adults. I think I've got useful opinions on the subject.

I believe there's a certain mindset that develops in musicians, that doesn't have much to do with the instrument or your age. The thing about being a musician is, you're always learning. I don't mean that in a hand-wavey kind of way. I mean you literally are always learning: New songs, new arrangements with different musicians and different lineups, new ways to address your own limitations (as you get older, physical limits can play a role in how you play/sing/perform something). There is no state where you're "finished" becoming a musician.

So...to be a good musician: accept that it is always learning and that you'll never be finished learning, and play whole songs with other people.

If you can't play with people, use a metronome or drum machine. Rocksmith is probably great, though, for muscle memory and rhythm; but, I noticed when learning to play drums with Rock Band that I immediately forgot the songs when the tracks weren't flashing by in front of me, so my "twitch game" skills were handling playing the game, while my "learning songs" abilities were seemingly switched off.

But, definitely play songs. From start to finish. If you flub a note, keep going, but play it slower next time, until you can play it without flubbing it. Chord books are a great way to get satisfying music without a lot of knowledge. If you teach yourself to sing and play at the same time, you can make real music all by yourself. People might even like it. And, work on one song until you're comfortable with it, reducing how much you rely on the chord chart each pass through (look at it every bar to start, then every other bar, then every fourth bar, etc.).

As others have mentioned, learning fast is not just an innate trait, it's directly related to how you feel about learning. If you view the learning process as a roadblock, it will be one. It's not a roadblock; it's what everyone does every time they play a new song.

And, to compare learning in childhood vs learning as an adult; I learned guitar as a child (well, I started when I was 12), and drums as an adult (I tinkered in my 20s, but never owned a drum set until my 30s and definitely wasn't able to do more than hold down a simple 4/4 rhythm before that). I learned drums a lot faster than guitar. Maybe I would have developed muscle memory better as a child, maybe I would have gotten more of a nuanced appreciation of rhythm or something, but I was able to focus my learning in very productive ways as an adult that I probably didn't have the mental framework to do as a child. It probably helped that I already had pretty deep musical knowledge on which to hang my new drumming skills...but, nonetheless, drums are a very different set of skills from guitar.

When I think back on my learning to play guitar, I very rarely learned anything until there were "stakes". If I was learning a song just for the fun of it, I wouldn't do it...I'd fiddle around, learn one riff, repeat it a few times, and then go play video games or something. If I was learning a song to play with my band, or to perform for an audience, it would stick a lot faster.