I've got a theory on that. More about the lack of continuity all together. In addition to the age thing, there are a lot of bizarre continuity errors.
The list goes on.
It's clear Louie doesn't care about continuity. But why? Because he's lazy? Forgetful?
I think it's because the show is supposed to be structured like a stand-up comedians body of work.
And when you watch a stand-up special, the comedian is free to pull anything out of his ass that will heighten the story he's trying to tell. He can just say "I've got a sister, she's kind of like this, and she told me..." whatever. And as we watch a comedian we aren't really keeping score on all this information.
We aren't stopping to say, "Wait a minute, you're talking about a girlfriend, but earlier you mentioned you were single."
Very few things are truly constant on Louie. The constant things are:
Those are the constants, and just like in stand-up comedy, all the other details just sort of come and go as convenient, in order to service the most interesting, and most humorous story.