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

I've seen toys with headphones that emit music in the stores and it says for ages 3 and up on the packaging. So I reckon, 3 and above will be okay? But then again, these toys were built specifically for kids so they have the child's safety in mind. In my opinion, it's more to do with how loud the sound is being emitted. It is not safe for anyone to blare loud music via earphones even adults so the same applies for children. If the child does use headphones, make sure the volume is at an appropriate level. Alternatively, you can look out for specific volume limiting headphones. These headphones will ensure that no matter how high your child tries to crank the music up on his iPad or computer, it will not be high enough to hurt him. Here's an interesting article that explains this device: http://www.howtogeek.com/222942/htg-explains-why-your-kids-should-be-using-volume-limiting-headphones/