The only way I can think of to do this completely anonymously (at least for the government and social media) is for you to buy a card in cash that has a little code on it. You'd need your ID to buy it, and you'd put your code into your operating system and things that demand age verification can ask the OS whether or not you're over 18. Alternatively, you can give the service your code to verify your age, but that would be less convenient and lead to a larger tracking footprint, so it likely wouldn't be used unless necessary