Eh, there are some very hard lines here for me especially due to several incidents of incest in my family. Good ol' Arkansas.
I do not like naming this type of relationship Dad n Son, unless its representative of a sort of bromance between a younger and older man. Things like being comfortable comparing dick size, swapping stories, horseplaying, masturbating around each other, showing different techniques. I can even go as far as mutual/shared masturbation, them having sex with different people in the same room, or even them running a train on or dp'ing some guy, like straight bros who are comfortable around each other and with their sexuality, but beyond that, I kinda see it as a technical foul.
Now, as far as in D/S, roleplay, etc., I'd much rather the older man be called sir, maybe even bossman, and the younger guy boy, laddie, etc. (in reference to him being young but still a man), even whippersnapper or stripling, hell, lol.
For me, I've always found older men hotter and the experience better. I can count on one hand how many guys I've been with that were around my age, and only one of them was younger than me. As I get older, I know I'll probably have to reevaluate how open I am with being with someone younger, but even being 31, if a guy is 18 - 20, its a no-go for me. I can go down maybe to 21. The only exception I think I can make to that is if the guy wants to bottom as I feel he would have a much better experience dealing with a "service top" than other tops that tend toward dom or are inexperienced. And as I get older, I think 30 ultimately will become where I draw the line, but, admittedly, it would also heavily depend on the guy's demeanor.