i would like to know where it is?

It's in Sochen Cave Palace... You need special requirement to open the door...

what's the special requirement to open the door then ... does it had to do with the ageworn key? if that ageworn key doesn't had to do with it then what does the ageworn key for?

I've been doing some leveling/farming there lately and I keep trying to figure out how to open the special door.

It tells you something about searching the footfalls of time. I've gone around the entire Sochen Cave Palace without finding the heart or whatever you need to open it.