TLDR: There may not actually be a caesura in the original poem at that line. The author certainly didn't indicate one by a punctuation mark. However, some modern poets do indicate caesuras, usually (in the cases that I have seen) by putting two spaces between words rather than by using punctuation. In modern poetry, you are allowed to do pretty much anything you want, so you can certainly put a caesura anywhere you please.
In the example that the OP quotes, the original did not have the punctuation mark ¦ that was used to indicate the caesura. It was introduced by somebody analyzing the poem.
The reason that the OP's source (which I assume is Wikipedia) says that a caesura is placed there is that nearly every other line in the poem has a caesura after the sixth syllable. Consider:
Till through the sleepy main ¦ to Thule I have gone,
And seen the frozen isles, ¦ the cold Deucalidon,
Amongst whose iron rocks ¦ grim Saturn yet remains,
Bound in those gloomy caves ¦ with adamantine chains.
Ye sacred bards, that to ¦? your harps' melodious strings
Sung th' ancient heroes' deeds ¦ (the monuments of kings)
And in your dreadful verse ¦ ingrav'd the prophecies,
The aged world's descents ¦ and genealogies;
I would say that there isn't a caesura in that line. Wikipedia uses that line as an example of enjambment, where the syntax ignores the break (at the end of a line or at a caesura) that we expect to be there.
Surprisingly, you might classify this line as an example of the French alexandrin ternaire, which is generally considered (despite occasional previous examples) to have been introduced into French poetry by Victor Hugo two centuries after Drayton used it in this poem. In the alexandrin ternaire, there is a caesura after the fourth and eighth syllables:
Ye sacred Bards, ¦ that to your harps' ¦ melodious strings
Victor Hugo used it as a means to break the monotonous rhythm created by the standard French Alexandrine, with a caesura after every sixth syllable (and which is more monotonous in English than French). Drayton may have been using it the same way, or he may just have not been able to find a suitable line with a caesura in the correct place.