How old do you think Harkon, Valerica and Serana are? also Garan Marethi, Vingalmo, Feran Sadri and Orthjolf.

Harkon, Valerica and Serana are supposedly old enough to be around when the cyrodiilic empire wasn't formed so possibly during the time period before the 3rd era and after the 2nd era, as for the others I have no clue.

A Cyrodrilic empire, and its certainly prior to Tibers. I'd say Hakron at least was around during the 2nd Era. With the chaos of the time period, its a wonder anyone could wrest control of Cyrodiil.