How long can a cursed child live if she does not use her inherent gastrean abilities?

Question

This light novel/anime might be years old, but I am still curious about how old a cursed child could get.

How long can a cursed child live if she does not use her inherent Gastrean abilities?

Answer 1

It is never mentioned exactly how long a Cursed Child has to live, however the corrosion rate increases even if a Cursed Child does not explicitly use their Gastrea powers.

The Cursed Children were constantly taking massive doses of corrosion-inhibiting medication to control the Gastrea inside their bodies, but it still only inhibited and did not stop the corrosion completely. Because the girls carried the inhibiting gene, they did not turn into Gastrea immediately like normal people

Kanzaki, Shiden. Black Bullet, Vol. 1: Those Who Would Be Gods (pp. 211-212). Yen Press. Kindle Edition.

From the passage above it is clear that Cursed Children are naturally immune to the Gastrea virus' corrosion, however without the help of the corrosion inhibiting medicine it is clear that they would succumb to corrosion even sooner which is even truer for Initiators who actively use their powers.

At first glance, it seemed like she had just caught a cold, but Initiators were protected from a variety of illnesses in exchange for the corrosion rate that bound their bodies, so of course that was not the case.

Kanzaki, Shiden. Black Bullet, Vol. 4 (light novel): Vengeance Is Mine (p. 125). Yen Press. Kindle Edition.

The Gastrea virus is always active, even when a Cursed Child is not actively using their powers and as such the corrosion rate will continually increase as the virus it "used" in a passive manner (healing wounds, preventing illness, fighting toxins etc.) Beside the passive "usage" it is also clear that the virus works on it's own, so while it is held back by the inhibiting gene and the corrosion prevention medicine the corrosion rate increase never stops completely.

Rentaro picked up the syringe that had fallen in the crack by the dresser. Inside it was cobalt blue medicine in liquid form. Realizing that she hadn’t taken her medicine, he became very sad. Nothing would happen if she skipped it for a day or two, but if she didn’t take it for a while, the corrosion rate of her body would gradually rise.

Kanzaki, Shiden. Black Bullet, Vol. 1: Those Who Would Be Gods (p. 95). Yen Press. Kindle Edition.

The piece above is after Enju ran away and while she will not be using her powers for fighting it is clear that her corrosion rate will still rise.

She was probably around seven years old, and she peered at Rentaro with a puzzled look on her face. Her eyes glowed red.

Kanzaki, Shiden. Black Bullet, Vol. 1: Those Who Would Be Gods (p. 98). Yen Press. Kindle Edition.

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Which meant that even Tina must understand that she needed to hide her red eyes.

Kanzaki, Shiden. Black Bullet, Vol. 3 (light novel): The Destruction of the World by Fire (p. 44). Yen Press. Kindle Edition.

From the above it is also clear that hiding red eyes is a skill a Cursed Child must learn, presumably (this is speculation, but probably valid) the cost of hiding red eyes means a slight increase in corrosion rate as the virus probably has to be suppressed in some way to achieve this result.

Now, the only corrosion rate ever mentioned (beside the over 50% for Initiators like Kayo) in the light novels is for Enju. At the start of Black Bullet Enju is 10 years old and she already has a corrosion rate of 42.8%. At this point in time she has known Rentaro for a year

“It’s been almost a year, hasn’t it?” she said. “Since you became a Promoter and met Enju.” “It’s only been a year,” he replied. “We’re still not even halfway to our goal.”

Kanzaki, Shiden. Black Bullet, Vol. 1: Those Who Would Be Gods (p. 33). Yen Press. Kindle Edition.

So, to actually answer your question...
Given that the oldest Cursed Children are only 10 years old, from in universe there is no way to know how old a Cursed Child can get, however if we speculate it is fair to say that no Initiator is likely to reach more than 10-12. At the end of Volume 2 Enju is given around 560 days to live (about 1.5 years) which would put her in that range. Cursed Children that are not Initiators are a bit of a wild card, but given that Enju probably started with a corrosion rate of around 30-38% it is fairly save to say that no Cursed Child would survive to be older than about 20. Even with corrosion inhibit medicine, which the non-Initiator children are unlikely to get, it is doubtful that any would survive into their late 20s