When they are older?
Male has a longer tail than the female and the female has a small concave in the top of her shell (to allow for the male to mount her)
as teni said males have a longer tail and their cloaca is further towards the end of their tail where females have shorter tails.
and not really sure on how old but i would guess over 2 years.
heres a pic of my female kreffts which is about 3 years old