The one good thing about living in Japan and using the ABRSM system is that I get to see more than one musical education culture at work. My enlightened conclusion is that neither those who say "must be achieving at a young age" and those who say "carpe diem" are completely right. Certainly not right in all circumstances, and for most of us, having only one iteration of each kid, hindsight is the only way to be sure that we got it right in even one set of circumstances.
Conclusion: you can only do what seems best at the time. You can ask for advice and listen to it carefully, and make the best decision possible AT THAT TIME...but you can't rewind. Luckily, though, you get all kinds of unexpected second chances in life

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For example, I had no idea that violin aroused such strong emotions in people when DS took it up. If I'd known, I would have forced him to play the banjo. But luckily I didn't know, and so, despite many wagging fingers, he has enjoyed violin in his own infuriating way.