As a teacher of music, this is a common question I hear. Every child is unique and while there is no one right answer, I can offer a few guidelines.
One of the first “games” I play with my younger students is to have them order the letters of the alphabet. This is a chance for them to show off their knowledge, build confidence and break the ice with their new teacher. I do this by giving them a stack of flash cards, each with one letter on it. By connecting this to the musical alphabet, there’s usually an “a-ha” moment. So if your child knows their ABCs, it will be easier to connect the dots to the musical alphabet.
Having a child who is passionate about music is probably the most important thing. The amount of time required to master these new skills and concepts is great. Has your child been asking about music lessons? Do you listen to music around the house? Does your child sing spontaneously? If so, these are all great signs that your child is ready for more musical challenges and instruction.
Fine Motor Skills
Many kids, especially younger ones, have difficulty controlling different fingers. With these children, I usually spend more time on singing, clapping and movement activities designed to internalize basic music concepts. With piano, these kids can play melodies with one finger. Other instruments may need to wait.
Voice is the instrument we already own. With all of my students, we sing, clap and speak out all of the songs we are working on first, to internalize their rhythms, pitches and phrasing. As we develop our voices, we can start to work on specific techniques like diction, phrasing, acting etc.
Piano is the easiest external instrument for anyone to learn. It does not require physical strength nor the building up of calluses or specific breathing techniques or lip tension. For all of my students, regardless of instrument, we spend some time learning the notes on the piano.
Guitar requires strength to press and hold down the strings. This gets easier the older the student. Check my school website for recommended half-size guitars for younger students.
I would recommend piano as the first instrument anyone learns and then if there is interest, to move to other instruments.
And if you want to get started right away, here's a simple course.