Note: There are times when the origins of phrases are not known. So what's usually provided, when this happens, are the popular or plausible theories that are around that talk about how a phrase may have originated.
In addition, quotes that contain a particular phrase may be from old newspapers, poems, or books. These are usually the earliest known quotes of a phrase being used in writing, and are meant to give you an idea on how far back in history an expression goes. So, for example, if I quote a book from the year 1809 because it uses a certain phrase, then the phrase is at least that old.
knowyourphrase.com - Common Idioms, Their Meanings and Origins
The earliest that I could find this phrase in writing with similar wording that it has today is from
a book called Once A Week, 1872:
"A correspondent of the same paper, who signs himself 'Octogenarian,' raised the question
of the date when 'There's no use crying over spilt milk' first came into proverbial use."
This expression was already a known saying at that time, so it obviously must be older, and indeed it is. There's a book by Hannah Maria Jones called Katharine Bereford; or, The shade and sunshine of woman's life, 1852, where a part from it reads:
"But it's no use fretting over shed milk."
Apparently, though, the saying is even older. It's said that James Howell, a historian and writer, used the phrase in a book called called Paramoigraphy (Proverbs), 1659:
"No weeping for shed milk."
This would make the phrase at least over 350 years old.
* My kids were upset because they had burnt their toast, but I told them that it's no use crying over spilt milk and to just make some more.
* Bill's car was heavily damaged in an accident earlier and was no longer functional. He was angry about what had happened, but he soon realized that there was no point in crying over spilled milk.