The issue with creating some hidden maturity heuristic for accounts is that it will be gamed just the same as any other, except that using age alone is the simplest heuristic to game. You can simply do nothing for incrimental periods of time and then begin testing aged accounts to roughly determine what the minimum age an account must reach to become "trusted".

Bot prevention is a very difficult constant game of cat and mouse, and a lot of bot operators have become very skilled at determining the hidden metrics used by platforms to bless accounts; that's their job, after all. I've become a big fan of lobste.rs' invitation tree approach, where the reputation of new accounts rides on the reputation of older accounts, and risks consequence up the chain. It also creates a very useful graph of account origin, allowing for scorched earth approaches to moderation that would otherwise require a serious (and often one-off) machine learning approach to connect accounts.