Despite the rumors, I still don't believe that the big three will be the only focus. I think it's more likely that Arndt really sidelined them, and Abrams just wants to make sure that they have more presence. This will still be the film where they pass the torch, but it will be a gradual transition. I'm betting the younger cast will still see equal or even more of the action, while the older cast deals with more of the social & political elements of the plot.

Also, regarding the age-ism, it's important to note that many people that were introduced to Star Wars back in the late 70s and 80s are still interested in the original cast and want to see them have one more shot at saving the day. I'm only 26 and I'm incredibly excited to see Luke, Han, and Leia get back into some action. We won't be seeing them in any mind-numbing prequel style acrobatics, but I'm sure as hell they can believably defend themselves during an intense skirmish. And the younger generation will be there to support them the whole way, while also drawing in the younger audience. You can tell from the casting rumors alone that this movie is going to be chock full of supporting cast members anyways.

I'm willing to bet that there will be two different stories going on. One following Luke, one following a younger character. The two stories will then tie together at the climax of the film. There is no doubt in my mind that both generations will have close to equal parts in the plot, they will just be juggling different issues.