Not because a child can play it (although a child can) but because it rings in the frequency range that older listeners tend to lose. One corner of the triangle is left open to keep the instrument from having a specific pitch and to allow it to generate ethereal, scintillating overtones instead. They are the secret of its glitter. When jangled with a metal beater, the triangle raises the energy level. It is very good for suggesting dance.
A steel rod bent into the shape of an isosceles triangle, this is a young person's percussion instrument.
Did you know?
The first prominent orchestral appearance of the triangle is in Franz Liszt's Piano Concerto No.1, from 1849.