Armin is male. There are good reasons why females voice young male characters. This may explain why a female, Marina Inoue, voices Armin in the original Japanese, BUT then there are good reasons why a male, Josh Grelle, voices Armin in the dub, see: this answer, where Grelle seems to say it's more flexible for a male to voice Armin because of potentially how far the characters could grow up to be.
At that time we didn't know where it was gonna go if there was ever going to be a time skip or anything like that. But just because of the sheer possibility of being with these characters for a while and having Armin eventually potentially grow up into a man, he wanted to have someone's voice that could cover the full age range. And that's one thing that at least I'm able to do.
So, what, the Japanese version regrets casting a female or at least something like they would've cast a male had they known how far the characters were going to grow up? Or, what, Marina Inoue can also do the full age range?