The magical yet mysterious door is the room where they can age as they went. It's called the Mysterious Age Room. While thinking on what age they wanted, the subject will ether progress or regress an age they wanted. The subject will tell the door on ether get locked to stay or open to get out of the room. The age will stay progress and regress while doing that.
When the subject said, "progress" in a room, the age will grow and grow while having unexpected changes and see what the future will look like. If the subject gets too old, He or she will risk losing a life and turning into ash inside the ash vase. The ash vase will still remain but the process of progressing age has stopped.
When the subject said, "regress" in a room, the age will shrink and shrink while having changes that did in the past and see what the past would look like. If the subject gets too young, He or she will risk unbirth as a mother will appear bringing a subject back into her belly. When the regression continues, A Mother will have to regress as well after oozing out the sperm of the subject and the cycle continues to loop without ever leaving.