Press Shift + Ctrl + Alt and type in "testingcheats true" or "testingcheats on" in the box that pops up in the left corner, and then after that type in "cas.fulleditmode". Then shift + click on a sim and it will open the family into CAS and you can edit anything about them! Just be careful, I think if you age up a child in this CAS mode it may break some family relationships? They may have fixed that, but it happened to me once.
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Because not all Sims are human.