This kind of relationship is not at all different from the older man/younger woman that is pumped into the brains and psyches of straight men. The Dad/Son/Boy, Coach/Player, Bear/Cub, Sir/Boy dynamic is a male version of the Man/Cheerleader, Teacher/female student dynamic and fantasy that permeates a lot of porn.
My man is 10 years older than me and I know a lot of couples where there is an age difference of more than 2-3 years. The age difference is not a big deal at all.
The other thing to consider is that age is not what makes someone a "daddy" or a "boy/cub/pup." Attitude and chemistry between two people is key.
As Indy has said, he looks like a daddy but is all boy on the inside. I look and act more boyish most of the time and definitely have my daddy tendencies. If I am around older, more mature men, I fall into a more cubbish role by default. Similarly, when I am around younger men, I assume a more mature, "daddy" type of role.
I also have a buddy who is the daddy in his relationship, even though his BF is 5 years older than he is (46-51). they work out together and are both in phenomenal shape.