Babies aged up without being the active household

The Bheeda baby was born and aged up to toddler when my sim lived in the same building, with aging on for active house only. But I got a few childless couples pregnant in an attempt to help my doctor get his promotion, and those have stayed babies for ages. I have always assumed being apartment neighbors was part of it. She hasn't ever aged any further though, and Billie Jang didn't age when my sim lived in that building for several weeks, it was just the baby to toddler transition.