Children as young as a few months old can learn various music skills, like song recognition or clapping. You can take your little one to music classes as young as a few weeks old. Even my 2.5-year-old can keep tempo, sing songs, and dance in time to the music.
More formal classes usually begin around age three. By age six, they can start learning an instrument properly, although they might need more time to get the hang of it. But it’s better to start earlier than later if they are truly interested.