I can't even begin to give you an answer, but would also consider other variables as important: level of interest of the child, your goals, your child's goals, the natural ability of the student, access to instrument/quiet/etc practice environment, frequency of lessons, is your child participating in band already etc.

I grew up playing and loving music, and each of my children has played multiple instruments - and the answer to this for each child + instrument + stage of life has been very very different.

So - total non-answer from me!

polarbera