Bismarck.Dracondria said:
»I don't think he's over 1000 years old though, Yamamoto should know who Ichigo and Isshin actually are if he's younger since that's how long Yamamoto has been the Captain Commander.
He's much older than 1000, Its just the Gotei 13 was founded 1000 years ago, and Yamamoto has apparently always been the Captain Commander (According to Bleach Wiki)
So, If Isshin was a captain at any point in the Gotei 13, Yamamoto should know. If it turns out Isshin is a Royal guard, there's always a chance his "Existence" would be wiped from the memories of those he knew so he could secretly protect the king without anyone knowing who he was/his powers.
Wouldn't explain how Urahara knows (of) him, But it would explain why no one else seems to know who he is, yet he has the clothing of a Captain, and was said to once be a captain?