What is the Age Regression Treatment?
it says exactly what it does on the page:
so you pick a zombie skin you've already unlocked in the past, and then you pick which stage of the progression you want it to be frozen on, and then that image will become your avatar base permanently until you choose to remove it.
this is useful because some people really liked making avatars based on some of the specific infection stages (for instance i think the creepy cow/deer skull that starts off Wrapture makes for a fun "companion" to design a matching outfit around), but in the past we would only have a limited time to make and enjoy that avatar as the infection would inevitably progress to another stage. this feature allows you to keep it frozen in that stage as long as you want, so you could make an outfit around it and keep it looking that way until you're ready to change.
For 10K sp Rachelle in shadowglen underground will regress an zombie strain that you have to one of it's earlier stages. Lets say you love the two cats in lab coats of stage 2 Transmewtation, but hate the final outcome you can re-set it for that. It will apparently still show as finished for biting, but be frozen at a stage you really liked.