It might be very difficult to pin point a particular "oldest reference," but naturally the English language is chock full of references to Shakespeare all over the place that are a part of every day speech. Phrases like "to be or not to be" (from Hamlet), "a rose by any other name" (from Romeo and Juliet), or even "method to the madness" (also Hamlet) are all Shakespeare references. These date back about four hundred years.

Older things you might consider references could be phrases like "Achilles in his tent," which is of course a reference to the Iliad, but I don't actually know how long that phrase has been in use in the English language. It's quite possible that the phrase only entered common usage more recently than Shakespearean phrases. If anyone knows if there are references that appear earlier and stay in usage to this day, that may be helpful (Beowulf, being an early work of English literature - Old English but English nevertheless - might be a good source).

Nothing ventured,nothing gained comes from Chaucer and remains in use today. That I think would be the earliest in recognisable English.

It has remained in consistent use since Chaucer wrote it in the 1300s, it appears in a collection of proverbs dating to 1546. It appears in the letters of Cadwallader Colden and those date from 1746 and since then you find it constantly used. It would be the oldest phrase in English that has been used consistently every since it was written.

Chaucer has a few others that also fit such as many a true word is spoken in jest which is again still used today.

As for pop culture well the Ancient Greek comedies were full of refrences to the culture of the time. Most famously in The Birds. Cinesias appears and his appearance in the play is mocking him and his poetry.

"cloud cuckoo land" is a direct translation of a phrase in Aristophanes play, The Birds. That's from 414BCE

Edit: as others pointed out, Homer is earlier (perhaps 850-1100 BCE, but consolidating aural tradition that goes as far back as the bronze age (before the bronze age collapse c. 1200 BCE). One phrase that still resonates today: Was this the face that launched a thousand ships?

I don't know about the oldest, but here are some old ones:

The expression "tying yourself to the mast" is a reference to The Odyssey. People can be referred to as "Cassandra," which is a reference to Aeschylus's Agamemnon and The Iliad.

Tasks are sometimes referred to as "Herculean" or "Sysiphian," both of which are references to Greek mythology. The term Oedipus Complex is a Greek play reference (as is Electra Complex).

Of course, there are also many biblical references that are still popular today (e.g., Sodom, splitting the baby (a reference to King Solomon), David and Goliath, Adam and Eve, etc.).

Perhaps biblical. You reap what you sow, the writing on the wall, my cross to bear, lone voice in the wilderness, old as Methusala, you are your brother's keeper, live by the sword die by the sword, house divided against itself cannot stand. Probably a ton more.

Edit: just thought of another unrelated one, I came, I saw, I conquered. From Ceasar's report on his conquest of Gaul.

It could quite be "Achilles Heel" taken from the Iliad when Achilles was shot with an arrow in the heel his only weak spot as that is where his mother held him when she bathed him in the river giving him his strength as he was a half mortal his father being human.