Children are fascinated by music, from birth. We use it to lull them to sleep, help them learn, and play. A lot of parents contact me regarding music lessons for their little ones, whose love of music is apparent from a young age. I encourage parents of kids of all ages to nourish their little ones love of music. Loving music and learning an instrument are two different things though. A child can love music, but not be ready for lessons.
First of all, tiny hands, fingers, and burgeoning coordination skills can be a huge obstacle to overcome. While a child may bang on drums or enthusiastically strum a toy guitar, transferring that to the fine motor skills and coordination used to play an instrument is a challenge. Secondly, children under the age of five lack patience and the work/reward logic needed to be able to practice consistently and for any length of time. What ends up happening with a child forced to take music lessons before they are ready? Enthusiasm is quenched by the difficulties of learning. Many kids can get turned off an instrument if introduced too early, because it's not what they expect and the FUN part doesn't materialize fast enough for a young learner. Also being unable to read, most young ones must learn by rote and ear, which may or may not work for each student.
My advice to parents: Ecourage your student's love of music by exposing them to various types of music, purchasing toy instruments, and having music time on a daily or weekly basis. That way, when they have the proper skills to begin taking music lessons, they will already be familiar with the process of music. Let them just have fun now, so they can have fun with lessons later.
What's the ideal age to start music lessons? That depends on each child but after 10 years of teaching, this sweet spot tends to be age seven for guitar, and age five or six for the ukelele (which is sized for smaller hands). Taking voice lessons can be very beneficial for young ones learning to speak properly, since voice lessons cover diction and pronunciation. I recommend as young as age four for voice lessons.
Lastly, if your child starts music lessons and after a month seems to be discouraged and discontent, unable to practice, then waiting another year might be better than continuing lessons that are not effective.