Notapushy you can train to a certain degree but there's a ceiling.
For example I am considered "bright". Went to a top school (scholarship). Achieved well etc. However although I was in the same class as my classmates, I was top of the class in (for example) History yet some of them were streets ahead in Maths over me. No matter how much time I'd have spent on Maths, I wouldn't have matched or surpassed some of those in my class. It wasn't to do with tuition at home either, as soon as a new concept was introduced to the class those girls would instantly get it, right in front of us all. So there was no skullduggery where they were secretly average but were tutored behind the scenes. They were just very naturally talented at Maths.
Even in primary school I remember I knew as young as Top Infants (Year 2) who were the "clever" ones in the class and who struggled. By 4th year Junior (Year 6) we all knew perfectly well who was bright and who wasn't, and it wasn't to do with tuition, and it wasn't because of who we'd been told was bright. We knew among ourselves.
A child can be tutored so far, but it would be very hard to take them to a high level of achievement and keep them there for their entire academic life without there being real natural talent. It would just be too hard.
As for music, probably any child could achieve some of the lower grades with input, interest and practise. But to get to Grade 8 takes years, dedication, real skill with the instrument and musicality. The expression you give to a piece. The technical skill you need to be able to perform pieces at that level. The examiners are not looking for "just" a perfect rendition of a piece, technically correct with no wrong notes (although this is necessary). There is timing, expression, musicality. Making that piece come alive. You have to have skill/talent to do that.
I am useless as sports so I'd never be a top sportswoman or even an average hobby sportist. Even if I'd been coached I might have been a bit better but I'd never be good, just average. We all have different born levels of intelligence, co-ordination, spacial awareness, finger dexterity, musicality etc. It can be improved on but above a certain level it's talent as much as tuition.