Glitch: So my family of four adopted a kid, and within one day the child as aged from 7 to 9, however the rest of my family hasn't aged at all! Any idea why?

Question: I've heard its around 4 hours our time for them to age one year. But do they only age after 4 hours of in-game time that you're actively playing. Or just 4 hours even when the app is off?

Thanks!