Rislav the Righteous has a deal more on the earlier history of Skingrad.
"Like all true heroes, Rislav Larich had inauspicious beginnings. We are told by chroniclers that the springtide night in the 448th year of the first era on which he was born was unseasonably cold, and that his mother Queen Lynada died very shortly after setting eyes upon her son. If he were much beloved of his father, King Mhorus of Skingrad, who already had plenty of heirs, three sons and four daughters before him, the chroniclers make no mention of it.
....
Children of kings are, of course, married to the children of other kings to bind alliances. The kingdoms of Skingrad and Kvatch skirmished over common territory throughout the fifth century, until they reached a peace in the year 472. The details of this accord are not recorded, but since we know that Prince Rislav was in the court of Kvatch six years later, as husband to Belene, the daughter of King Justinius, it is fair to make an educated guess that they were married then to make peace.
This brings us to the year 478, when a great plague swept through all of Cyrodiil and seemed particularly concentrated in the independent Colovian West. Among the victims were King Mhorus and the rest of the entire royal family in Skingrad. Rislav's only surviving elder brother, Dorald, survived, being in the Imperial City as a priest of Marukh. He returned to his homeland to assume the throne."As for Hassildor's age, don't take his looks to mean anything. Vampirism completely halts the aging process. He may have been in his 30s/40s when he contracted Vampirism (which makes the infected look a lot older, so subtract the 20 years it adds on - plus there's the 'pre-infection' portrait of him), but how long has he had it?
A few things - the publican of the West Weald Inn tells the player that Hassildor has ruled all her life, and she looks to be in her late-forties/early fifties (to me).
Toutius sixtius says that "Father and the Count were best of friends.". sixtius himself is a haughty Imperial whose father I doubt would be friends with someone he knew was a vampire.
His wife was also infected, but refused to feed. When she is cured of Vampirism she dies immediately; 'age catches up with her' so to speak - likely why Hassildor doesn't take the cure, it's a death sentence. So we must conclude that he is older than the natural lifespan of Imperials.
He is known to be a 'powerful wizard', so his lifespan is unusual in itself.
I reckon he's about 100-ish years old, maybe a bit more. Emperor Uriel was 87 (likely greater than the average lifespan - he was emperor, after all), and is referred to as an 'old man', for a frame of reference for Imperial lifespan.