Earlier quoted context omitted.
"Ago" is kind of agglutinative. In english it isn't used by itself. It's pretty much a suffix even though by convention it is it's own word.
It's more like a preposition that exceptionally comes after the noun phrase instead of before it. (Is there another thing like that in English? I can't think of one, though there's one in German: entlang) Etymologically it's a past participle: three days agone (gone) => three days ago
Called a ‘postposition’. And that’s precisely what ‘ago’ is!