When can we let our child use an earphone/ headset to listen to music?

I will avoid my kids to use headset or earphone before they turn into adulthood. So far, my kids used headset once during the flight and they do not like it, so I don't see the point of introducing them to my kids also. Long term usage will damage their ear drum so I am not in favor of it. Normally we just switch on CD player and enjoy music openly.