This has happened enough times now that I'm starting to wonder if it's a thing™. I'm a young gay guy and I've had several middle-aged guys approach me while I'm out at the clubs with friends and also at restaurants/cafes etc. just to say a version of this- I used to look just like you, I was beautiful/hot and now I look like this ::points at self::, it'll happen to you too".
I just think it's the most bizarre thing because I don't know what their goal is, I just reply with a puzzled look on my face "Is that so? well, thanks for sharing". I wouldn't think anything of it, but it's happened like 5 times already which is annoying.