Sidonia no Kishi: Daikyuu Wakusei Seneki Episode 7

[10th_man_down]

Originally Posted by chriskim019

Reminds me of an anime I've seen before with a similar concept of people

growing up as unisex until the age of maturation and choosing their gender as

either male or female with slow process. Can't remember the name though.

However, that anime had spiritual rituals for them to do so and not just

naturally like in this one.

Not sure if this is it but http://myanimelist.net/anime/879/Simoun is close to that description except they all start as females instead of unisex.