Lake of Youth

In the deepest caves, after escaping the Warden, fighting all mobs, and escaping certain death, A pool of glowing water, which grants eternal youth to pets. Because, lets be honest, I was kinda bummed out when my pets grew up, and wished they would remain small.

If a lake/ potion is an unviable option , maybe a rare item , found only in places like Woodland Mansions, or Sunken Ships, which can be used on only one pet (Like a nametag)