For Those Playing Legacies: When Do You Switch Over Focus From Previous Generation to the New?

I usually start to focus on the heir once the parent has completed their LTW or the traditional way when the heir has finished Uni and is now a YA.

Currently, I'm playing a game with rules I've made up (not ready to share the rules yet, just testing things out) for a legacy. In it, the heir doesn't begin control until the parent hits their LTW AND reaches level 10 in their chosen career. Kinda tough, but makes for interesting gameplay.