Myleus Schomburgkii: When does one see the difference between male / female?

[arowana]

Hi, got myself a group of 15 Myleus Schomburgkii. Size of all animals is about 12 cm.
I know one can the difference between male/female by looking at the dorsal fin. At this size they look all the same to me. I can't imagine that they are all females. Can anybody tell me at what size/age one can see the sex difference? Thanks!

 

[AhKwaB]

I thought you were suppose to go by the shape of the anal fin....

Females are more like a sickle shape and males a stepped hook.