Babies aged up without being the active household

Definitely what @luthienrising said, and I would stress that he should definitely re-check all of the households in My Households to see what their "played" and "unplayed" status is. After the NS patch update I kept having households automatically get changed to "played" even though I never had played them.
It stopped once I turned off all of the NS options other than "adopt a pet" for all of "My Households" and it never was a problem for "Other Households".