I think it gets significantly harder to learn new things as you age. It could make sense though like its significantly easier to learn a new language as a child than an adult, something simillar could apply to sheet music also i guess where they are naturally more intuined with its language vrs you who has to kinda use a dictionary for time to time because the "normal" way for you to speak has been so different for so long. But for your child, its another form of expression almost. Its really good to get kids into music while they are young imo