How does anyone know whether a family is engaging in that time-honored tradition of passing down accounts from grandfather, to father, to son, to child, and their posterity, in perpetuity?
Seriously though, unless you have positively identified the person who created the account in the first place, you have 0% chance of knowing whether it is the same person using it today.
Gamers sell their high-level accounts all the time. It would be a simple matter of economics that the Discord users with the oldest accounts sell them to 12-year-olds. Likewise, accounts are shared willy-nilly, whether or not that violates the rules. And accounts can be stolen or compromised, if you're really hard up.
Seriously though, unless you have positively identified the person who created the account in the first place, you have 0% chance of knowing whether it is the same person using it today.
Gamers sell their high-level accounts all the time. It would be a simple matter of economics that the Discord users with the oldest accounts sell them to 12-year-olds. Likewise, accounts are shared willy-nilly, whether or not that violates the rules. And accounts can be stolen or compromised, if you're really hard up.