One of my goals is to learn to play the piano well. I started late in November 2014, and have been practising quasi-regularly, averaging around 3 hours each week. This shows is where I'm at:
This is the result of approximately 33 hours of practice, and it took around 20 takes to record this video - previous takes always had mistakes. Although playing at this level is far below what others believe when they hear anyone say "I can play the piano", it is a phase every piano player had to go through to become good.
Whenever someone demonstrates amazing skill, there is a tendency for us to feel as though "I wish I could do that". But not many of us try, at least not for long enough.
The barrier to entry may be that "I don't want to sound stupid", "I don't feel motivated", or whatever else. Perhaps that is why children are in a better state of mind and life to acquire skills. They don't believe they sound stupid when playing scales* for half an hour, multiple times a week. They practice for that long because they are disciplined to do so (typically by their parents). How many twenty-plus-year-olds are willing to begin at that very basic level? Without someone else to discipline (read: enforce) us to practice, how many of us still do?
* playing scales are the equivalent of singing "do re mi fa so la ti do"
post script: I started by playing scales, and I still play scales as warm-up every time I practice. I don't always feel motivated; that is why discipline is important.