I used to be a performing musician myself for many years and I have a formal musical education. The answer to your question depends on the type of music and activity. It takes a lot of time (much more than the famous 10'000 hours) to learn these enormous skills as e.g. a classical pianist and to be able to perform a classical work with the virtuosity and endurance that is expected today. If you don't start as a child, you lack the time until you reach professional age. Moreover, children can learn many things more easily than adults. In pop music it's completely different; you can be successful there without mastering your instrument, if you look good and have a good stage presence - or another characteristic that just happens to be important to a large audience. A young audience is naturally more likely to be attracted by young idols. A big portion of luck is needed in any case.