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For us, with both kids, 2 and a half years old is when they absolutely refused to take any instruction. I'm so happy we got our potty trained before then. I can't imagine trying to teach them anything like an instrument.
But maybe if you have a friend or know a teacher who likes kids who can sit with them every now and again, you might get some results. At around 4-5 years old, my daughter started being able to learn from other adults.

She's about 2 and a half, and yeah, instruction is a non-starter. God, I wish she were potty-trained. I'm not really worried about direct instruction, though. She'll copy me, or she won't, and either way she'll hopefully have fun.

Don't forget about rhythm instruments. Electronic toy drums are good, and maybe buy her marracas when she's a little stronger because they're really just rattles for big kids, so she already knows how to use them.