The quote you provide would certainly suggest that 90 would be extraordinary in ancient times - I doubt Pliny would have devoted much time and effort to the subject if it wasn’t (and it should be noted that the chapter in question also “cites” several people living over the age of 200, so maybe we take those numbers with a grain of salt or two (or 200)). Hell, it would have been extraordinary in 1900!
Living to ninety is unusual today, as well as then. But Ramesses II (the Pharaoh known as Ozymandias) lived to age 90 (and Pharaoh Pepi II Neferkare is said to have lived to 100), so it was hardly impossible. (Yes, high-placed Kings were better nourished, but I pick Ozymandias as example simply because ancient dates are known exactly only for top aristocrats.) Gruffydd ap Cynan, King of Gwynedd lived to age 82, despite that he spent many years in prison and healthcare was poor in 12th-century Wales. These few examples are just off the top of my head.
Anyway, an age of 80 or even 75 might work adequately for the scenario. Is a John the Beloved living to age 80 unlikely a priori? Sure. But lots of things are unlikely. Jesus had many devout young followers; we could expect several to live to old age — and those oldsters would know that their personal connection to the Christ was important. They would be eager to share their memories with someone like Polycarp. “John” might have been an ordinary disciple who promoted himself to “the disciple Jesus loved” in his retelling. Does this impact John’s utility as a source, and as evidence of the historic Jesus?
Probabilistically, should we assume that John the Beloved lived to age 80? Heck, I don’t know if the “disciple that Jesus loved” even existed. But to extrapolate from “unlikely” to “almost certainly not” is just plain unjustified.
Searching through my VERY disorganized library in an (unsuccessful :() search for a different book, I stumbled upon a book I read decades ago: Jesus: The Evidence by Ian Wilson. It may be worth a read for those interested in thread topic. I’ll mention just a few items from the opening chapters:
[ul]
[li] John 1:2-3 is very similar to Manual of Discipline 11:11 from the Dead Sea scrolls. There are many such similarities between the two books.[/li][li] “Give alms” in Luke 11:41 makes little sense; the same story in Matthew 23:26 has “cleanse.” This is explained by the similarity between Aramaic ‘zakkau’ (to give alms) and ‘dakkau’ (to cleanse). Therefore Luke was translated (poorly) from a hard-to-read Aramaic text.[/li][li] That Matthew was also translated from Aramaic is shown by parts of the Sermon on the Mount which, when translated back into Aramaic become distinctively poetic. (Similar to translating ‘On the bridge at Avignon’ back into French.) Bishop Papias (b. ca 60) states that the Apostle Matthew compiled sayings of Jesus which others borrowed.[/li][li] Wilson quotes C.S. Lewis insisting that John is reportage not myth. I don’t see either Wilson or Lewis at Google Books, but Lewis is quoted near the bottom of this page.[/li][li] Mark was written in Rome where Jews were hated. This explains why the Jews are made the “bad guys” in the story, Romans the “good guys.”[/li][/ul]