According to Ngram, anyway.
The vast majority of English speakers seem to have no idea what the word means.
Now why is that?
UPDATE: After reading some of the responses:
As a noun.
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.Sign up to join this community
The learnèd word coeval, sometimes written coëval and formerly spelled coaeval or coæval, has fallen on hard times for one simple reason alone. Now that the clerical class is no longer required to study Latin, the constituent morphemes of the word are no longer accessible to them.
Because they no longer immediately know that eval meaning “of an age” is from Latin aevum, neither do they know what coeval means, and so it falls quickly into the forgotten desuetude all such cranberry morphemes (quick! what does cran- mean?) are destined to die in.
The same fate has befallen aeviternal, longaeval/longeval and even primaeval/primeval, and even mediaeval and mid-eval have become simply medieval with no one having any idea why in the world we refer to the Middles Ages as “medieval”.
In short, the illatinate masses cannot automatically recover the aevus or aevum portion of these words that gave them life and meaning. To anyone with any Latin, however, all these words have an obvious meaning related to a certain age, just as aeon does. Notice when each of these words entered in English, per the OED:
eval, adj. (1791)
coeval, adj. and n. (1605)
coevity, n. (1641)
longeval, adj. (1598)
longevous, adj. (1652)
primeval, adj. and n. (1653)
In comparison, medieval from medium aevum is a recent borrowing that barely showed up 200 years ago, some 200 years after those in the previous set. Back then the educated still studied Latin and Greek, so its meaning was still apparent to them. Today, they do not, and so the connections have been lost.
Interestingly, the far more commonly used word age ultimately derives from the same Latin roots as all these words do, namely aevum and aevitas, but it has suffered the gnawing erosions [sic] of time until it is no longer recognizable as such.
Coeval is a useful word with a straight-forward meaning. Yet I have never used it.
I realise that I say contemporaneous when I might as easily say coeval. Ngrams suggests that this is because contemporaneous replaced coeval around 1840!
Why did coeval fall out of favour? It is attractively short but, possibly, a more daunting pronunciation challenge....?