Captured slaves that get certain girlids can spontaneously age when added to your household.
In essence the following code in the "NewSkills" section
/* OLD AGE */ <<if _npc.girlid is 317 or _npc.girlid is 321>><<set _npc.age to random(55, 62)>><</if>>
gets run AFTER adding captured slaves to the house. This code is certainly intended to run as their portraits look fairly old, but it should happen when they are captured or at least show up during inspection before deciding whether to sell or add them. Having old grandpa "surprise" you with the fact he is not actually 23 years old just seems a bit silly.
Edit: I'm still running 0.8.4. if that matters