While I don't have a problem with Uhura being alive in the TNG era, the idea of her as some sort of spymaster sounds like a bad fanfic to me. Head of Starfleet linguistics or something like that, or retired, fine. But shady intelligence agent? Ugh.
As for character ages, I'll make my obligatory mention of Admiral Archer in STXI, a century before TNG's time. And although some say he was a descendant of Jonathan Archer, the writers intended it to be the same guy from Enterprise, and cited McCoy's cameo in "Farpoint" to justify it. Perhaps that can be used (retconningly) to pinpoint roughly when human lifespans shot up. IIRC, Dr Phlox gave 100 as the upper human lifepan during the Enterprise TV show.
Oh, and Morgan "Number One" Primus is another TOS survivor into the TNG era - although she's certainly a unique case.