The first 600,000 people have not entered their ages on here, so that could be an obstacle to filtering on Roll20's side. In the meantime if you're a player, you are able to ask the GM and the other players how old they are, or, look for games where the GM has ruled that only 18+ can join. Some GM's will write that requirement in their pitch so you can look for that. You can also add to your Player Profile, "I want to play games with people in the age range _____ " if you are GMing -- what you can do is use the free-text Description fields as GM to post whatever requirements you like. "Must be over age 18" or whatever stipulations you want to put. This is commonly done on here. GM can also ask the players to fill out your application and tell you before you accept them in the game: "How old are you? What do you think makes a good roleplaying encounter? How well do you know this game system?"