Why is it "the day is young", not "still early"? What is the history of the phrase?

Mark Halperin’s article on the Missouri Congressman and Republican Senate nominee Todd Akin’s gaffe in August 20 Time magazine ends up with the lines:

“So far, not publicly calling for Akin to leave the race, as Scott Brown did. But the day is young.”

I am interested in the phrase, “the day is young,” which I understand means it’s still early to tell what will happen next, or the end result.

Google Ngram shows this phrase emerged in 1840, and its usage has sharply declined after peaking during 1910–1940.

What is the history of “the day is young”? Do you say 'the day (or year) is old'?

By the way, we have a popular saying, “The day is long,” in Japanese. But it has a rather positive meaning of “You can do (achieve) still a lot of things before the day is over,” for example, “Let’s have another drink, Taro. The day is long,” after having one.

Can’t I use “The day is young” to this effect in English?

I don't think this idiom is particularly common -- I don't read or hear it much -- but I think most English-speakers would recognize it.

It has both a literal use and a metaphorical use. You can say, "We'll get to that later -- the day is young" meaning there is still time in this literal day. You can also use it to refer to a longer period of time, as would appear to be the case in the quote you cite. I doubt the speaker expects the issue to necessarily be resolved within 24 hours, but the implication is that it will be fairly soon.

As to saying "The day is young" at 5:00 to indicate there is still time to go out for a drink or whatever ... I think English-speakers would be more likely to say, "The night is young", a slightly different idiom.

"The day is young" corresponds to "the hour is early" or better still simply "it is early".

To me "the day is early" would be slightly unusual, but might suggest the early part of a longer period, such as a month or year.

"Young" and "early" are semantically extremely similar. The use of one for the other in English is not too surprising and usually easily understood.

Both "the day is young" and "the night is young" go back more than a century as idioms or truisms. Consider this excerpt from "Miss Landon's Poetry," in Blackwood's Magazine (August 1824):

Good heaven! somebody will say, what is the meaning of this rigmarole cock-and-bull sort of nonsense? Do you take us for Peripatetics? By no means, my good friends, but there is no need for hurry. The day is young. Hooly and fairly goes far. Take the world easy. Blow not your horse in the morning, and you will be the farther on when night falls.

And from Arthur Reed, "Drinking Chorus," in The Cambridge Review (May 23, 1889):

Our griefs shall break like a bursting bubble, / Our fears like a curl of smoke be gone; / So drain a bumper to sink all trouble, / And pass the punch as the night wears on! / Then drink all round / Till our heads be drowned— / The night is young and the bowl is crowned!

Here is an Ngram chart of the "the day is young" (blue line) and "the night is young" (red line) for the period 1800–2008:

The chart indicates that the "day" version came into currency before the "night" version did, but it also shows that, since about 1950, the "night" version has been considerably more common that the "day" version. Still, both expressions continue to appear fairly frequently in published works.

Of course, when people say "The night is young," they generally mean "I'm still having fun and I don't want to go to sleep yet"; whereas when people say "The day is young," they may mean "There is lots of daylight left and lots of time to do what we need to do" or "We've awakened near dawn and the whole day lies ahead of us" or even "Okay, it's after midnight, and it may be inaccurate to say that the night is young—but if it's only 1 or 2 or 3 a.m., we can certainly say that the day is young."