Unless you've been living under a rock for the past year there is Caitlyn Jenner, 66, who recently transitioned.
If your focus is celebs, there will be very very few. Celebs certainly do not represent the norm. There was a day, until a decade or so ago that the far majority of transitioners were older, as in 40-50. I suspect the mean age is still up there, but that's a wild guess