As in all things of this nature, attractiveness has to be an important deciding factor. Each guy has idea of what is attractive, so it will vary from individual to individual. I'm also going to assume when talking about attractiveness, we are talking about looks. Defining attractiveness can be esoteric, but I'm going to limit it to looks for this discussion.
Granted having a mistress/dom is playing in a different park than a "normal" encounter, but still, the "mistress" should be attractive in the eye of the man. Having said that, this is a specific type of encounter which requires the lady to have "special" skills for this type of fetish. A lady's performance, style and abilities have to weigh into the equation, but a guy should be able to have his cake and eat it to
For me, I'd be willing to go with a 7-8 in the looks department if I knew the mistress/dom experience would be outstanding. Normally, I try to see providers whose looks would for me be 8-9-10. So, it would be not that big of a difference in my book, but still a "sacrifice" I'd be willing to make. Granted, I'd still be looking for that 9 or 10 in the looks department for a potential mistress/dom, but only if she is really good at the fetish.
As for age, it is a factor. An older mistress/dom would have more experience in the game than a younger one. If I were to seek a mistress/dom, the starting age would have to be 30 with a preferred target of 35-40. I think for a guy wanting this type of session, he'd want to have it with an experienced mistress/dom. I know some enjoy "teaching" a less experienced (younger) mistress/dom, but I think a guy is probably not the best "mentor"/teacher for this fetish. I think a younger girl wanting to be a mistress or dom should learn from a more experienced lady. BTW, I'd be willing to be the guinea pig for that experience