Youtube’s search engine is pretty good and you can usually pull up someone’s username if it exists. You can sometimes replace the channel name with the player’s username, but due to youtube’s method of channel naming, this doesn’t always work.
Sometimes a youtube URL will be formatted like this:
https://www.youtube.com/c/DominionServer/
but other times it will be formatted like this:
https://www.youtube.com/channel/UCIs9_nhkuPuBP2L9MEWb_cg/
Twitch is pretty similar to youtube. You can check the About page, and also any videos or clips they may have posted. The About page can give you a name, location, age, and links to social media. Sometime’s there is a photo so you can see if they are under 16. Otherwise you can check some of the streams. A lot of people use face cam on Twitch.
Twitch links are formatted like this: https://www.twitch.tv/chushberry/ so you can replace the username with the minecraft username and see if there is an account with the same name.
Once you find a youtube/twitch channel that you think belongs to the whitelist applicant there’s a few things you can do.
About page. This will sometimes have their age. For example, “Hi I’m a 12 year old from the USA who is making videos about LEGO.”
Example of a Youtube About Page showing age, and social media links
You can also check the Youtube About page for the joined date. If someone says they are 16, their join date will reflect that. Young people cannot manage a youtube account, so a 16 year old’s join date would sometime in the past few years. It’s always possible a parent set up an account for them, so this information would not be a conclusive way of checking age, but taken in context with other information it may be useful.
Example of a Twitch About page showing age and social media links.