I think they could use Quinto, Nimoy was 34 when he played Spock in
The Cage, and age on screen, especially under heavy make-up, is easily vague enough to cover the difference between Nimoy-Spock's age in this time frame and Quinto's current IRL age - look at the actors who portray 'teenagers' on the CW

. Quinto is such a good Spock that I think it would be a shame to recast again. I don't buy the 'they look different because it's an alternate timeline' anyway, they just recast the roles. Just like Tom Hardy doesn't look like the guy who played Ensign Picard in
Tapestry.