Smallest size to sex plecos?

[Mr. Johnson]

I was just wondering when the sexual characteristics start to show on a pleco, in general.

I know that body shape can be a factor. Head shape can be a factor. Odontodes can be a factor. Crescent shaped pectoral fin can be a factor.

I have notice that it's very hard to tell a female and a subdominant male apart. So, in general, when do the above mention sexual characteristics really start to show on a zebra both in age and size?