A technological challenge

The kittens are starting to wander back toward my office and lab.

I should block the hallway against kitten infiltration.

Problem is: Piggy, the elderly cat, sleeps in my office and needs to get to and from the living room and kitchen.

So: what sort of barrier will allow an arthritic old cat to pass, but stop energetic little kittens?

Clearly, I need a semi-permeable membrane, permeable to everything except kittens.

Or, some sort of robotic Maxwell's Demon, using machine vision to differentiate between kittens and not-kittens, and open and close a gate accordingly.  And I need it, like, an hour ago.

Or some sort of trained rodent, but that would intimidate Piggy more than it distracted the kittens.  And, again, not in place by an hour ago.