I can know if your child is ready to take music lessons when he start getting interested in the type of music cause he/she doesn't want to take lessons to the type music he does not like
If you sense his/her actions are inclined to musical instruments or even she would go a day with listening to music, then definitely she's more than interested enough in attending music lessons. And if he/she is shy and won't agree even if deep inside you know she likes, then encourage her to do so and in that way, you'd help her in achieving her dreams and so that her hidden talents would also be discovered!