It may be possible for a 50 year-old to run a sub-4 mile; I tend to doubt it, but given that a sub-4 mile is literally not as difficult as it used to be (due to shoe and track technology), some elite athlete who also happened to be one of those people who seem to age really well (sort of a double lottery-winner in life) and who also happened to still be motivated for some reason, just might be able to pull it off. Maybe.
But if somebody does it, it won't be due to "being able to ignore the expectation of being past your prime", as you suggest - as if every aging athlete so far in the history of sports has been stupid and gullible, and didn't really need to call it quits when they did, but could have kept competing at a top level to age 50+ if they had just realized that age is only a number.