This is a great question! The best age depends on the instrument and the teacher.
For young children around age 5-7 (depending on their development and maturity), piano is a great instrument to start on, even if they later want to pursue other instruments. They will learn to read music, which can then translate to whatever instrument they choose after.
For kids 7-10, guitar, ukulele, violin, drums, and voice are great options. As long as they have the strength in their fingers (and lungs), any of those instruments are available.
For ages 10 and up, wind instruments, including flute, clarinet, saxophone, trumpet, French horn, and trombone, are all viable options. Students have to have the lung capacity to produce a quality sound (and also, we don't want to damage their teeth growing in), so we wait until about this age before starting on wind instruments.
If you have a question about what age and instrument is right for your child, please feel free to reach out and we are happy to discuss your options.