I started playing when I was a teenager. Through the ensuing 30+ years, I never quit playing, but there were periods of more intense learning.

I can't say that learning anything now is any more difficult that it was back 30 years ago. The big difference between now and then is availability of time and energy! :).

Everybody is different, but one thing I have found about myself is that it's more than muscle memory for me. A lot of the repetition for me is not just embedding the movements into my muscles, but the notes and the music into my brain. I found I would often struggling with a solo not because it was technically demanding, it was because I didn't have the music in my head.

I play covers. Some solos I do are improvised, others based on the original. Regardless, I have found that just listening the original songs I am learning over and over again (on my commute to and from work) helps a LOT...much more than I anticipated.