Minecraft Forums have been around for a long time and it is one of the primary places people check out to talk about minecraft. Its a forum and so there are posts and profiles that can be checked.
Sometimes if you search a username on the Minecraft Forums you can get an age confirmation, as a lot of younger people will apply to servers there and give their IGN and age.
Or alternatively there will be a location and age in their profile that you can compare to the whitelist. Sometimes you can find social media links in there too.
PlanetMinecraft is a community where people share their creative minecraft efforts. Things like skins, maps, texture packs, data packs and more. There is a forum and people also have profiles they can post to.
PlanetMinecraft does not really have an easy way to search for players and the member list is disabled until you are a member of a high level. Your best bet to search PlanetMinecraft is to use Google.
Search google using:
Username site:.planetminecraft.com