Once one character has completed chapter 1, the legacy unlocks for all characters, regardless of level.
However, to get the legacy in the first place, one character has to complete chapter 1, and that happens after all the class quests on Alderaan, and then some specialty quests.
The game's milestones are generally based on story progress, not level.
Yes, but you can't do the quests required unless you've achieved close to the level for each world (or have help), so most people need to be around 32 when they complete Alderaan. Yes, you could be level 50 before you complete chapter 1, but most people would not wait that long for their first character to complete its legacy.