Old enough to vent?

[mbuna man]

How big or how old do they need to be before I can tell the sex of my current sub-adult Mbunas?

[creek]

You can vent as small as ~2", but I wouldn't bet the farm on the results unless you've got experience. As long as they're juvies and you don't have any particularly troublesome species/individuals in the tank, you can watch and let them get a little bigger before you go stressing your eyeballs out trying to vent tiny fish. It should be a lot easier around 3".