Why did it matter that they were older?
It doesn't and I didn't say it did.
Riker being red-faced and bloated speaks more to things other than age. Such as, perhaps, a fondness for distilled or fermented spirits, but definately a lazy attitude regarding his big screen appearance.
As for Picard, that had more to do with bad lighting and poor choice in camera setups and blocking. Patrick Stewart is still a fine actor and has been a commanding presence onscreen in other things since Nemesis.
And what kind of drama or loss can the audience feel towards Data's "sacrifice" when they introduce his special-needs replacement earlier in the movie. This film was a poop-bag of cynical filmmaking for which they should all be ashamed.
So my criticism is centered on things other than age and on things that easily could have been corrected. There was zero reason for Nemesis to be as incredibly bad as it was other than a clear-as-crystal lack of interest in producing a quality work.