I can't answer whether there's an advantage psychologically, or whether there's something about being 5 that means you learn more quickly than when you're say 20.
But there are circumstantial advantages :
I learnt to play guitar at around 19-20. I became what most people thought was "pretty good" quite quickly.
The reason : obsession. It was a summer holiday between college and university, and I was utterly worn out mentally. Learning guitar was just the kind of therapy I needed - so I threw myself at it- several hours a day- and it worked.
I would guess that children (say 5 - 17) have enough time on their hands to do this, and are often encouraged to study music especially if they show an aptitude, so it's kind of built-in. If playtime is used playing music (because said kiddie wants to) then the learning will at a huge rate.
I think for a lot of adults this opportunity isn't so easy. There are other commitments - studies, work, family etc which mean while you can be an enthusiastic student, you can't dedicate hours at a time, or days, to it. Or more accurately : You can, but perhaps a lot of people just don't.
One other difference in the way kids learn is repetition, which is why a lot of them are so good at video games : they'll try something and if it doesn't work, they'll repeat it again and again until they succeed or see a change - like a tricky level on a game, or mastering some element of playing an instrument. An adult is more likely to get bored/frustrated sooner, probably again due to a time constraint or other outside influences, meaning it may take longer to get there.
That said, I don't think either of these points directly imply that kids learn in a way that makes them inherently better at something than if starting as an adult- it's just the way they go about it, and the opportunities available.
One other factor is the obvious one: time. Two people learn guitar .. one starts at 5, the other at 20. When they're both age 30, the 5 year old has 15 years more experience.