It all depends on dedication. Muscle memory doesn't stop at any certain age, but it tends to take longer to get it to click. I went through a similar thing as you. I played in punk bands when I was young after my dad showed me how to play a few chords. I didn't care about lead guitar, or theory at all. After my kids got older, and my wife asked me to find a new place to live, I decided I wanted to actually learn how to play. I had a lot of time to practice - literally 2 hours a day after work, and 4-6 hours on weekends playing the same diatonic, and pentatonic patterns until not only did I play them well, but I actually understood how to use them. And, I spent countless hours on sites like these learning licks, and riffs. A lot of tips from this site. Anyway, 2 years after the obsession to be a good guitar player, I'm pretty good - Not a virtuoso, but moving in that direction. I guess, what takes a 15 year old 1 year is like 2-4 when we get older. And, I just want to be great because i love it, and that's what I want for myself. It's passion bordering on obsession, and even if I'm playing alone, i want it to be the best it can be.
Dedication and practice routine.....