I thought Luke and Leia were different ages too. So long as it’s not explicitly mentioned, that detail can change too. Also I’m a bit confused as to why Luke and Leia had to be twins in the first place, and not just siblings.
If they weren’t twins, how would you explain that Vader didn’t know his wife was pregnant not once but twice? He didn’t know about Leia.
Good point, honestly I haven’t fully thought this through, but I also don’t think he’d not have to know his wife was pregnant twice. He could have been well aware of Luke’s birth, nothing in the first two movies implies he wasn’t (George even altered the dialogue in ESB to change that!), plus that would make the “Your father wanted you to have this” not an outright lie. He may have been led to believe Luke had died in a Jedi purge.
Silverwook’s explanation alleviated my confusion, but I still think making Leia a Skywalker was a mistake.