Searching for a HPSS fic that goes like this.
Severus Snape called in a favor for Harry (who is now an Auror) to take care of his younger self. Snape got hit by a curse in the final battle that has him randomly switching ages. I believe that Snape is 16 while he is living with Harry.
There was also this funny incident where Snape makes a mess of the kitchen experimenting while Harry is at work.
I know this fic is archived here in the HPSS section but so far I haven't ran across it. It's probably buried clear back there. I know I was going through the HPSS section backwords when I ran across it *blushes*.