How Not to Summon a Demon Lord follows Takuma Sakamoto, a secluded friendless nerd who loves to play the MMO Cross Reverie. In classic isekai anime fashion, Takuma gets sucked into the game, turning into his character, the Demon Lord Diablo, in the process. Once there, he finds he has been summoned by Rem Galleu and Shera the elf and must work out how to get home while helping the people who brought him into their world.
Working out someone's age is usually pretty simple. However, How Not to Summon a Demon Lord makes it slightly more troublesome due to the strangeness and magic that exists within Cross Reverie's MMO world. Determining the popular Rem's age is especially difficult, as she's far from what she seems at first glance.
Rem Galleu is said to be 14 years old. However, she is later revealed to be the host for the soul of the Demon Lord Krebskulm, a curse she inherited from her mother. The Demon Lord Krebskulm is an ancient being, having been sealed in Rem's family line for at least three generations, of which Rem is the most recent. This means the soul is, at minimum, 100 years old. Of course, this argument hinges on debating if Rem and Krebskulm are counted as the same being or not. While Rem has held the soul for her entire life, she and Krebskulm are not bound together permanently. If Rem was to die, then the soul would be released and Krebskulm would be reborn.
However, during How Not to Summon a Demon Lord's story, Takuma does learn of a way to separate the two safely. The ritual works but only partially, meaning Krebskulm isn't yet at full power. She can only achieve her actual demon form when she thinks Rem has died, making this bond exceptionally confusing. While Rem and Krebskulm can exist in two physical bodies, their forms are still technically intertwined, making it hard to tell where one of them starts and the other ends.
The conversation of Rem's age becomes even more confusing when you consider How Not to Summon a Demon Lord's MMO setting. In the world of Cross Reverie, did the past really happen, or did everyone appear fully formed in their current state? Their memories of the past could be real, or merely fictions made by the game's programmers and implanted into the characters. There's also the issue of how real the world of Cross Reverie is. Is it truly a living, breathing world, or does the game's code still hold some sway over both the characters and their lives?
Rem's age is an intriguing riddle that helps fans understand just how traumatic her curse and possession must be. To be stuck carrying the soul of a world-destroying tyrant is not an easy thing to deal with, no matter one's age. However, the fact that the soul has been passed through Rem's family -- and with Rem and Krebskulm able to be separated -- means that using the age of Rem's physical body, 14, is likely the most accurate way to answer the question.