I have two parents (plus one of their siblings) living together in a house: The parents have 2 kids- one infant, one toddler
The toddler was supposed to age up today but wasn't giving the manual Age Up option, eventually the day was about to end and my toddler wandered outside. He walked all the way outside to the back of the house, and aged up automatically.
He aged up straight into Teen, then Young Adult, then proceeded to have a Cardiac Explosion. I couldn't do anything to stop it. I'm attempting my first legacy playthrough and its very disheartening to have this happen unprompted.