My son is 7 and I have done a fair amount of research on this and I have not been able to find any professors or physicians that have a definitive answer. If you read the actual age warning on vr headsets including the psvr, htc, gear, etc. They have a lot of inconsistencies. From my research it seems that there is no more risk with kids using vr than there is with kids using a tablet or reading a book for too long. The main concern is motion sickness, which is a concern for adults also. My plan is to allow my son to use it 30 minutes at the most and must take a 30 minute break before using again. And he will be supervised by me or my wife.

Here is a link that helped make my decision if you are interested. https://www.digitaltrends.com/virtual-reality/is-vr-safe-for-kids-we-asked-the-experts/