I'm not exactly sure which patch broke this, but it was within the last few months. When I load a household of four young adults, the YA sims' order is different than in CAS, and even giving them all...
Ideas
If you use mccc you can use it to set their exact ages as just (for example) 1 day apart from each other in the order you want them and then enter/leave cas and it should re-order them as oldest on the left to youngest on the right in live mode. It's the only way I've found to manually control the order.
Console players can't use mods.