Meaning of AGO in English
■ adverb before the present (used with a measurement of time).
ME ~ , ~ne , past participle of the obs. verb ~ 'pass', used to express passage of time.
When ~ is followed by a clause, the clause should be introduced by that rather than since , e.g. it was sixty years ~ that I left this place (not it was sixty years ~ since I left this place ).
Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004