I've searched but have not found any information about the origin of the phrase "in this day and age."
Has anyone researched this? Why not write "today"?
Many of the earliest examples I could find are in religious/legal contexts, but not exclusively so. Most of the early examples I found were from the early-to-mid nineteenth century, though the construction itself may predate it. Here's the earliest, published in 1826 but supposedly from a 1769 edition:
These are the blessed people that have this knowledge; and all that have it, know how they. come by it, and how they became so, having been the temples of the un-clean spirit. They know the temple that was defiled and polluted, cleansed and purged, and the unclean spi+ rit and his works cast forth. These. are a watchful people against all that would defile the temple again, and these are the true temple worshippers in this day and age, and this is the temple in which everyone speaks of the Glory of the Lord, of the power of the Lord, and of his wondrous works wrought therein, even in his temple not made with hands.
Here's another early example, from 1832 .
In this day and age, noone who is blessed with health and a common share of that indispensable article, good sense, can have an excuse for ignorance, and that hyena monster with her numerous retinue scattering desolation over the earth, ought not to find a resting place in our land.
As for 'why not write today?', English has a variety of constructions that it likes to use, and 'in this day and age' is one of them. 'Today' also feels flat compared to the full expression, which sort of evokes a feeling of expansive reference to all of what's going on in the world these days.
Why not write today?
In this day and age — wordiness notwithstanding — could be considered more precise than today. Compare:
I prefer to work at home today.
Today, I prefer to work at home.
In this day and age, I prefer to work at home.
? I’m eating bacon and eggs in this day and age.
With the right context or intonation, today could possibly be used for in this day and age, but in this day and age can’t be used for the 24-hour period we call today.
Here’s the OED entry for this day and age:
P6. With another noun.
b. this day and age: the current time. Originally and chiefly in in this day and age: at the present time; nowadays. Cf. sense 14a(b).
Source: Oxford English Dictionary (login required)
What about all those words, though? According to the OED, day can mean era and age can mean era. Aren’t we just saying the same thing twice?
In rhetorical terms, it’s a kind of tautology called hendiadys1 (“one by means of two”). As a rhetorical device, it’s used for pacing and meter and emphasis, so it’s not surprising to find it employed in sermons and religious tracts.
What is the origin?
The earliest use of day and age I could find was in the 1659 publication Mr. Tillinghast’s Eight Last Sermons ... The third edition.
Mr. Tillinghast, having died in 1655, would have uttered these words some time before that.
You can find plenty of other early usage examples in a Google Books search for
”day and age” for 1700 and earlier.
1. There’s an interesting discussion of hendiadys in God’s Word for Our World, Vol. 1