I can't remember the finer details, but even if it was some sort of medicine that doesn't exist in the real world, being 33 and looking much younger on feminizing HRT is not unrealistic. Testosterone is what ages the skin and makes one look older than they actually are, so not having much of that in your body for however long is going heal up a lot of that damage—especially if Annie is maintaining a skincare routine and avoiding a sunlight exposure. Heck, Annie already looked young before beginning to transition, so that helps, too.
Indeed, from the beginning of the story, Roger was presented to us with a youthful appearance, only hidden by his beard.
Even Miss Jones, the teacher, tells him: "Mr. Rogers, have you ever been told that your face is very boyish? What looks very young for your age?"
Even Rogers admits it by saying: "Yes, someone has told me once, but with the beard I hide it a lot. Why do you ask?
Even Miss Jones's plan was for Rogers to look like a young teenager, but she did not explicitly indicate whether he was a 16-year-old boy or girl, since even his height helped in that process.
Even when he shaved his beard, he was surprised to see himself in the mirror with that appearance: "I didn't remember what I looked like without a beard, I really look like a kid."
And even his own wife admits that she confuses him with a brat, telling him: "...Look at you, you took off your beard, you look skinny, I swear if it wasn't for that suit, I'd think you were a brat."
And then with all the machinations Miss Jones did at Rogers, she looked like a 16-year-old girl, albeit a bit tomboyish but still very young-looking from the start.
As you can see, Roger already had that look, the experimental drugs and hormones only helped speed up the process to meet the current Rogers, or rather Annie.