Mark Hamill will be about 63, Ford will be about 73 and Fisher will be closing in on 57 by the time cameras roll on principal photography. As much as I revere their characters we have to face facts: they're not spry action heroes anymore. Harrison Ford barely made it through the last Indiana Jones film without looking like he needed to take a nap and rub on some Lanacane. Han, Luke and Leia will no doubt be leading and major characters in any storyline set 30 years after Jedi, but I sort of have my doubts that there are going to be many glitzy, frenetic action scenes for most of them.
All that doesn't matter if the writer finds a way to include them and believably account for their importance in the story besides spouting some analogy of "I'm to old for this shit" or some Jedi Master wisdom just because they're old compared to the spry, young action cast.
In fact i think Hollywood could use some more mixing of older actors with younger ones especially because there are so damn few young outstanding actors (i mentioned Jennifer Lawrence already which was no joke.. i think she's a brilliant actress despite her physical beauty.. it's the rare example of talent and good, marketable looks). For example a most recent example would be the movie Red where they take nearly exclusive old actors like Bruce Willis, Hellen Mirren and John Malkovich and turn them into a funny action bunch including their age.. it's a damn good and funny movie (and the sequel will be released later this year).
If they manage to do that with Star Wars i'll be one happy camper.
Oh.. and regarding this Carrie Fisher thing.. all three principal cast member are currently apparently only in an advisory role or something like that. They have been invited to meet with the main writer but have not signed contracts yet. What that means and if it's true is up to anyones guess.. if it's true and they get offered contracts to appear in the movies i doubt anyone will turn it down.. they are not exactly on food stamps (Ford certainly not, Hamill is doing ok with voice work and Fisher has/had a career as a writer) but i think they'll all welcome the chance to be in the spotlight again.
If the script holds up and doesn't turn out to be some stunt casting for the sake of it i'm all for it.