Technically, the pick is by order of sims *in CAS*, which is not necessarily the same as the UI if you're playing with an overstuffed household or one that's been split/merged or had deaths ... AKA, anything past first gen. The UI still shows by age, but if you get the household into CAS, you might find it in order of toddler, young adult, elder, child, adult. Back when we had the bug where only three sims being able to celebrate holidays, it was the first three by CAS order as well, so a lot of us learned real fast which sims got sifted to the front. But in a vanilla, first-gen, straight-out-of-CAS household, yeah, both of these things should match up, but it's still in order of CAS order, which will remain the order they were created in, even if one hits a birthday cake early and the other drowns herself in Vitality Wine and they shift by age order in the live mode UI. Until you mess that order up with split/merge/death/etc.
