I don't see why Mariam would have forgotten her assassin skills. It's a physical memory on par with riding a bike--even if you don't remember where you learned it, you still know how to do it. Odds are she rose up through the ranks quickly, so she could have lost her memory at 8 or even 9 and worked her way up that year.
Keep in mind she was also experimented on by Abyss, who could have done any number of things to her in terms of physical/mental enhancement and demonic possession. Look at Ney. She's in her teens during the series proper. She's even younger when she's sent to the palace as a spy during Twiright Prank--she's actually not that much older than Allen and Riliane, who were themselves young children at the time.
Considering the context it's entirely plausible that Mariam was a preteen assassin who was too quick to get punted away by her enemies, as it were. Her skill is not strength but speed and knowing where to stab people.