I'm taking part of a RP in the forums. And the character I'm playing as is related to Ocato. And I need to know the year Ocato was born so I could make my character's age accordingly. So are there any sources describing Ocato's DOB? If not, what year would you guess?

No, there are not. And given the nebulous information regarding Altmeri aging, and the lack of information regarding his age in the games, fixing a DOB is impossible. You can make it up as you see fit, really.

It can be safely assumed he was born in Firsthold, given his name, "Ocato of Firsthold". But there's a dearth of information leading up to when he became a member of the Elder Council. And even that date isn't known.

He was appointed as Imperial Battlemage in 3E 399, beyond that not much. Based on his appearance, he's likely a bit younger than Barenziah, so I'd guess he was probably born right around the time of Tiber Septim.

I was going to say the same. The first century of the Third Era seems plausible. Although he could be a great deal younger than that.

His birth would probably have taken place sometime after Tiber united Tamriel. Had he been born before Tiber's conquest of the Second Aldmeri Dominion, I'd imagine he wouldn't want anything to do with the Empire.

Then I could at least guess he was born in late Second Era at best, though I can't see him being pro Imperial before the Third Era.

@Dovahsebrom, I'd say this then. If he were to "join" the Imperial government, I would think he would do it as like a double agent to anti Imperial groups in Summerset. Otherwise, shortly after Tiber's conquests.