Hello!
(I can't speak English very well because I'm not English.)
I started playing flute when I was 5 years old (Soprano, alto, tenor, bass&one other. Don't know what it means in English...->

).
When I was 11 I started learning to play oboe.
Since one year I play the piano, the clarinet, this thing->

and a little bit organ.
I was always interested in music and instruments. So my parents had no problem to interest me in it.
And as long as children aren't intrested in music and instruments from their own you can't do anything against it. You can ask them if they would like to learn an instrument. If they say no you should leave them alone.
You asked for a appropriate age to interest a child into playing an instrument, didn't you?
There is no-one who is too young to be interested in music. As parent you should support it already during the pregnancy. Listen often to the music, sing a lot, play instruments etc.(even experts say it helps) When the child is born you can sing with the child (perhaps children can't speak as a baby, but they try to imitate other people). When it is older (1, 2 years) you can give your child a little xylophone or rhythm-instruments like a

.
Later your child is used to the music(possibly this sentence is rubbish) and want to learn instruments.
Is it possible, that this text doesn't make sense?
Englischhorn