What do you mean by "learn music"? While starting to play an instrument at that age is probably great, it's probably way too early to learn the theoretical underpinnings or the history of western music. I also know of many people who started much later.
When my eldest daughter was touring the University of Michigan's music program, they asked all the potential students how long they had been studying. Many of them said five to ten years. However, my daughter said she'd been only studying for a year and a half. (These kids were all high school juniors.) The other kids looked at my daughter as if she wasn't a serious musician, but the people from the school said that FOR SINGERS, it is extremely normal to not start serious studies until their mid-teens. Prior to that, the voice just hasn't developed all that much. (It's also normal for them to not be able to get serious classical gigs until their mid-20s. Again, the voice isn't there yet.)
Of course, she did have a Montessori education which (among other things) pushed reading music and Orff Schulwerk, so she knew how to read when she was very young.
I wouldn't tell anyone that they're too old to learn music. And for the extremely young, make sure they're having fun and they have an instrument that fits their body.
