Mage the Ascension is, at least in my eyes, a game of exploring the paradigma of the characters played as much as their struggle to achieve something in the war for reality. The Dark Ages variant mainly differs to the modern age Mage, that the Technocracy hasn't formed and pretty much won yet, and the backdrop is pretty medieval.
So, let's look at the factors that shape the player character we have here:
- Noble on the level of baron/baroness.
- Dead father (so there is something to inherit).
- No male siblings (so they must inherit after the common primogeniture/gavelkind succession laws in Europe).
- In case of a male character: just no older male siblings.
Now, let's look at the proposed time Mage Dark Ages is to be set: 13th Century. And then the Campaign intended is set in 913 Leon, which is either a Kingdom in Iberia or a Viscounty in Brittany. Both are glorious places: In France the Normans, neighbors to the NE are powerful and the crown offers lots of intrigue, in León the Reconquista is fully running.
There are tons of conflicts that could have resulted in writing the parents out. Or an Intrigue. Or bad fortune like highwaymen, illness and a falling rock during castle construction. The result counts, as you (GM) want the character to be Conveniently an Orphan, or at least a half-orphan.
If you feel the sudden need to remodel the map of Europe to your liking... I would grab my trusty Conquer & Marriage Simulator CK-2 and start early (You'll need either Old Gods or Charlemagne), then switch to observer mode and run the game for some years till 913. That way you just created all the other local rulers and map borders you probably might need. But that's just me being silly.
Now, we made the character an orphan somehow. What to do to help them to get to adventure? How about looking into the trick box? Oh, there is the classic Advisory Council that does the day-to-day work as long as the new Baron(ess) is a minor, right? What did make Vlad III. of Walachia the hater of Islam that would earn him the names Drăculea (Son of the Dragon) and Țepeș (Impaler)? Oh, right, he was the hostage of the Ottomans. And Cordoba did not shy away to try to secure themselves in similar manners when they had the chance. But all in all, all these are coming of age-stories.
The Good Council
A good advisory council (that might contain a mage-teacher for the young mage) will keep the boring part of ruling away from the young lord/lady, so they can study, but at times ask them to oversee some matters to make them an effective and good ruler when their time comes. This can drive an entire campaign! It's a little bit of Hogwarts combined with politics.
The Bad Council
Of course, the council also could be rotten to the bone and only a few servants are on the side of the future ruler. Thrown into the turmoil of politics and forced to swim on their own, this too could be the drive for a campaign. This is much more intrigue-focussed and less Hogwarts... This premise can take inspiration from the visual novel Long Live The Queen!.
Well, why make the child an Orphan, if we can write the parents out of the picture in a different manner too? A larger, more powerful Kingdom might have demanded the future ruler to be educated and protected in their capitol, which is pretty much polite for taking them as hostage and trying to indoctrinate them as friends. In the case of Vlad III, this was a total failure, but then again, it offers a nice backdrop: We have the teachers of the emperor's children & hostages, who are very likely to be powerful people in politics and possibly Mages themselves. There are the other children of the emperor, who have agendas that might be more in the direction of fun and adventure. And then there are the other hostages, probably older ones, that might want to lay claim on the lands that the character one day will rule and thus want to take early actions to take them out of the picture. So many possibilities!