Looking for a science fiction story I read years ago, about teleportation gateways that the user finds are making him younger when he travels through them.
This is not much to go by, but in Exile's Gate, the fourth of the novels about the heroine Morgaine by C. J. Cherryh, the viewpoint character (Margaine's ronin servant, Nhi Vanye) is reverted to a fixed age every time he passes through a teleportation gate. Morgaine had programmed the gates to do this, to heal his injuries.
Since the plot of the whole series involves Morgaine traveling from planet to planet through the gates and closing them behind her, this is a potentially important plot point. It means that Vanye will not be able to outgrow his approximately twenty-year-old appearance as long as he continues to travel through the gates. However, since the novels ended after Exile's Gate, the issue was not pursued too extensively.
This might possibly be the Damon Knight story "Ticket to Anywhere" (1952.) It has a network of teleportation booths that don't make you younger, but do temporarily stop the aging process while you're using them. You can read a copy at the Internet Archive, and it's been anthologized a number of times.