At the age of 8, Mozart was already writing his first symphony. Stevie Wonder was already singing at 11. It seems to me that postponing music lessons (until the child is older) can be a bad idea. Is there a recommended ideal age?
There is no "right" age to start a child into music. It should be when they show an interest in it. If you are playing the piano and your child comes up, you should begin there. I am a big proponent of getting children into music as soon as they show interest.
Learning is better when the child is interested, but it's also true that it's easier to absorb what you're learning at a younger age. A decent age could be six to seven years old when the child can start conversing well.
Maybe it depends on the instrument that you'll be teaching the child. Some are just easier to handle, even for adults. Kids can only get harmed, or the musical instrument might only get damaged if it's too much to handle.
I don't think you're ever too old to learn something new, but man, I sure picked things up faster when I was an adolescent. I think that quickly tapered off by the time I hit my 20s, so I say start them young.
Exposing the child to different hobbies or music will help answer that question. The desire to learn is more important than their age, in my opinion. Giving them an idea of what's there will surely help.