Error when aging up - Skip child age

On one save, I have 2 families I flip between and both ended up have a set of twins. The first set of twins were set to age up from toddler to child but one of them skipped child completely and aged right to a teen. Then the second families twins were aging up from toddler and both skipped child age and went right to teen. Used cas.fulleditmode to de age them so not too big of an issue. But did find it strange. Anyone else having this issue with their sims or sets of twins?

note: playing on Xbox