If Cumberbatch is Khan, then just don't give me any explanations in the movie for why he's different than TOS Khan. He's Khan, and that's it. I can take it.
(I've finally started catching up with "Sherlock" whenever I find it on. Cumberbatch's Holmes reminds me a lot Gregory House. The pace and direction of the episodes reminds me of the show, too.)
Part of the joy of Trek for many was growing older with our heroes. Having Kirk and the others age just as the actors did set Trek characters apart from how most other action heroes are treated. Every Bond actor is on the clock, for example, and everyone knows it. Every now and then, the franchises are contemporized, too (like "Sherlock"). It's expected. By comparison, the Trek reboot almost came out of the blue and shocked many people. Some not in a good way.
It's the way of franchises, and it finally happened to "Star Trek". Version 1.0 had a good shelf life. I doubt we'll ever seen Daniel Craig as Bond at age 62. I think it's unlikely we'll ever see Batman or Superman get their AARP cards in the mail, either. We may never even see Kirk grow old again.