I wouldn't recommend VR for anyone under 12-13 or so. Eyes are still developing at that age and there's a possible chance of screwing up focus with extended VR use.
This source says otherwise: "ophthalmologists agree there is no reason to be concerned that VR headsets will damage eye development, health or function." https://www.aao.org/eye-health/tips-prevention/are-virtual-r...
It seems like there is just a lack of data regarding the impact of VR usage on eye development and as a result people are being rightful cautious.