I've always wondered how much our muscle memory skills decline as we get older. And I'm only 26 so I can't really answer that! But I didn't start playing guitar until I was 25 and I'm actually learning pretty fast. But that's because I follow this rule: "The key to adult learning is repetition." And I like to add something to that for guitar: "The key to adult learning is A LOT of repetition."
But there's one other factor that matters just as much as the "quantity" of repetitions: it's the "quality" of those repetitions.
So it's entirely up to you. What are you trying to learn? Basic chords? Lead guitar? Rhythm? Are you playing on an acoustic or electric? What kind of music do you want to play?
Your answers to these questions will make it easier for me to give you some suggestions on how to accelerate the process.