Are the ex-parasites in Darling in the FranXX immortal? In the final episode of the anime, Nana and Hachi (who used to be parasites in the past) claim that they're immortal:


  We don't age and we're stuck in the middle, neither adults nor children. We have but one duty: to reach out to humanity's future and continue to watch over it. ~Hachi


But just a few minutes later the conversation between Futoshi and Ikuno seems to contradict it:


  It's all thanks to your research that us ex-parasites' accelerated aging is being kept in check. ~Futoshi


What information have I missed? Are ex-parasites immortal or mortal?
There is a difference between parasites and adults. It's known that adults don't age anymore because of APE. Since Nana and Hachi worked for APE we can conclude that their technology to make people immortal was also used on them.

Consequently, every parasite can become immortal. But in the last episode the ex-parasites decided to stop using magma energy which is necessary for achieving immortality. Since these people, being clones, age faster than humans, Ikuno was desperately trying to find a cure for that accelerated aging.

While not being able to find a complete cure for that condition, she still finally found a way to keep it in check which is more than enough for the affected people to live a normal life.