It's harder...the skills come slower with age (that trait often referred to as cognitive plasticity) but, *other factors being the same* I can't think of any reason age would prevent reaching a high skill level. If, at a young age one had the aptitude to become accomplished, that aptitude should be accessible at a later age.

Here, http://www.neatorama.com/2011/06/10/5-musicians-who-had-to-relearn-their-craft/ , are some stories of musicians that learned to play their instrument *again* after suffering injuries. One, Jerry Garcia, had to relearn basic motor skills after a five-day coma.

An even more eye-opening example is the jazz guitarist, Pat Martino. "Pat Martino made a remarkable comeback after brain surgery in 1980 to correct an aneurysm caused him to *lose his memory and completely forget how to play*. It took years, but he regained his ability,..." http://www.playjazzguitar.com/pat_martino.html He was 42 when he resumed playing.

Of course, other-factors-being-the-same is a pretty big qualifier. Sometimes other factors hinder, sometimes they help. That said, I'm progressing much faster than I did when I was a kid taking lessons and I'm well beyond 35...it's faster precisely 'cause my other factors aren't the same. :)

IMO, age slows down attainment, it doesn't prevent it. Yes, you will suffer the limitations of an adult beginner but how far you go is determined by you and how well you control those other (limiting) factors.