I'm not yet at the stage of finding DS a teacher as he is only 7 months old, and who knows, in a few years he may well prefer lego to music (though at the moment he is very keen on music and recognizes tunes — probably much like most babies).
Anyway.
Was talking to 2 friends about music lessons. Friend 1 has 3 kids, the 6yo started recorder this year, the others are 5 and 3 and more interested but apparently too young. Friend 1 played cello to semiprofessional level before kids. She thinks go with the child, some 3 yo kids are going to be ready for lessons, some 8 yo kids might still be too young — but she also says the sooner they start, the easier it is to learn everything.
Friend 2 has 2 kids. Has never played an instrument and doesn't like classical music, but is a developmental psychologist and thinks kids should learn music from age 6, no earlier. Her 7yo DD learns flute but isn't interested -possibly because she never hears classical music. Her 4yo DD would probably have the attention span, determination and interest to be learning now, but mum says she absolutely shouldn't start until 6.
Meanwhile I grew up in a demographic where kids started piano, singing and very basic theory at 3 or 4, second & third instruments at 5 or 6. Anything later was seen as too late for anything other than just doing it for fun (nothing wrong with fun, but don't expect to catch up with your peers or be a musician).
I suspect going with the individual child is sensible, looking at attention span, perseverance, interest, dexterity etc. But is any age just really way too young?