I overheard the chagrined page telling his knight, "I'll just heed to you once more, good Sir."
No. The more natural phrasing is "It will never be like it once was."
You just did. But if you'd like another example, here you are: The subversion of the once-powerful government caused chaos throughout the land.
I would like my chastity back, bringing with it the innocence I once owned.
His musicality lets him pick out tunes, just by listening to them once!
Yes. The convicted person loses their right to appeal deportation action.
We were in New Orleans just before the flood.
Here is a sentence I once found in a book: "Then I saw a Lone wolf, just standing there, howling in the wind."Hope I helped! :)
A poet once described the sea as being like a vast desert, only wet.
Once he had brought in a new bed and a desk he realised just how little room there was in his room.
It's not an idiom. It's just a sentence -- to be young again means that you are once more younger in age. This can be literally -- like in a science fiction time travel story -- or it can be figurative -- like your mind is young again.
"You once met the prime minister." is a correct sentence.
He was a careful man who did not like to take risks is an example of a redundant sentence. A redundant sentence is one that says the same thing more than once as in not liking to take risks and a careful man.