Kitten-Like actions in an adult cat

We have a mixed calico cat kayleh who is about 5 years old. She has recently made friend's with one of our other mix's Trouble, whom has never liked anybody, not even us.

Last night Kayleh jumped up on the bed and curled up next to Trouble. Trouble allowed this, and there still doing that now, and Kayleh is constantly mewing like a kitten would, and is even trying to nurse.

Trouble is acting very protective of her as well. Trouble had one kitten... but soon lost intrest in her baby... she never treated her baby like she is Kayleh

Is this a big problem... or.. just a stage?

Aww, that's just two cats being best of pals.

Aww, I guess that can happen.

They just seem to be acting more then just friends though... I mean its like Kayleh forgot how old she is and became troubles kitten and trouble became her mom.