How many years are their in an age? Also how long ago was the Battle of Red Mountain?

I don't think theres, some kind of limit of when one ages ends and one begins, its all about big events, third age started with forming of the empire by Tiber Septim and the begining of his dynasty, and it ended with Death of last Septim, and partial fall of the empire, so its kind of hard to tell, but I guess some Lore buff will correct me

There is an interminable number of years within an age. Basically, an age lasts until someone important enough decides to declare a new one.

Battle of Red Mountain was in the First Era, year 700. So that is... 3750 years ago. Skyrim reckoning.

http://www.imperial-library.info/content/tamriel-timeline

"Age" isn't really defined as it is. "Era" varies; the First Era was 2920 years long* and ended when the Akaviri Potentate declared that the death of the last of the Reman line meant a new era; the Second Era was 897 years long and ended with Tiber Septim becoming Emperor, the Third Era ended in 433 with Martin Septim, the last of the Septim Emperors and we do not know how the Fourth Era ends yet.

Before the first era was the Merethic Era, very few historical records survive from then, it is usually thought to be 2500 years long and is counted backwards. Time has no meaning in the Dawn Era, which preceded the Merethic.

Note that as well as "Eras" we have "kalpas"; this is a much longer and stranger denomination of time and it is usually thought that the end of the kalpa is essentially the end of the world.

*there were 1008 years in the First Era during which a Dragon Break occured; as such it is impossible to measure time or record events from then. Usually it is best to consider that time stops at 1E1200 and carries on in 1E2208.

A new age begins when the person or party in power says a new age begins. It might be 3000 years, or 400, or 4 million.

What the these good gentlemen (and ladies if there are any) above me said.

Thanks for the answers. I meant to say era's not age's.