Most people don't do focused practice over time because when you're actually forcing yourself to learn and get better at the edge of your ability it's hard.

I think people get better in the beginning when they know nothing, but once they know a few things most people just plateau at the level where they can play the couple of things that they know without forcing themselves to get better.

Children mostly are forced to do it, with all the advantages and disadvantages of that process.

All this is to say in the majority of cases you can trivially catch up if you're interested in learning a new skill.