We can't possibly answer this because we don't know... REALLY, WE DON'T... what role "old Spock" is playing in the film.
Is he involved in the "action?" Or is he essentially a narrator? Is the film supposed to be a linear adventure where it's his perspective we're seeing (even though they jump around through time)? Or is it set in the past entirely, but merely has Nimoy in the closing scenes in a "leap ahead" bit? Or is it framed by Nimoy, telling a story of his past to his protege (a Romulan)?
WE DO NOT KNOW. Do we?
How will it end up? Well, I'll give you my personal opinion... ie, what I would do if I were making this film.
My personal opinion is that Spock is very old when we first see him... not dying yet but clearly not too many years left to him. It would be implied that this is in the early 25th century, and Spock is well over 200 years old. He's talking someone, who we don't see clearly.
This younger man is wearing a uniform that is recognizable as a Starfleet uniform, though not exactly one we've seen before. He's facing his Command School trials, and is questioning his right to lead. He's questioning how he can ever match up to the great leaders he's studied. And so, he's asking Spock, his mentor, about this.
Spock relates the story of James T. Kirk, the greatest LEGEND in Starfleet history... and one who he has personal knowledge of. We get a brief flashback to Kirk's history... to his birth, of his learning of the death of his father when he was about 12, to his decision to join Starfleet. This is all in very narrative form, with Nimoy's words over the scenes.
We then return to the "present," and see Nimoy's face. He then brings himself into the story... and his face dissolves into Quinto's face, as he leaves his parents to go to Starfleet (with his father's great disapproval).
The story then becomes linear... but not static. It moves forward for the most part, but with "jumps" forward to different timeframes." With each period being there to tell a pertinent part of the story Spock is relating... one that takes place throughout Kirk's life.
When the story has been told, we return to Nimoy as "old Spock" finishing his tale.
Only then do we see the full face of the Starfleet Lieutenant who Spock has been mentoring, and realize that this Command-track Starfleet officer is a Romulan.
We've got a new character, tied to the past, but available for new, ongoing adventures... and one who would DEFINITELY be a break from what we've known before.
Anyway, that's what >I< would do.