I am just wondering on where I can get age thing so I can get to done
Age verification is currently limited as it’s still in beta essentially. Certain VRC groups have a link to provide in order to age verify, which then can be used to test 18+ group instances. The official release of Age Verification will take some time, my best bet is the first or second quarter of the upcoming year (2025)
You can sign up to get early notifications when the Beta becomes available by visiting the VRChat Age Verification page.
For more comprehensive details, I suggest checking out the post by Tupper: Age Verification FAQ - Official Notes on the VRChat Ask Forum.