Why is it that with wrasse the boy fish only turn into boys after the first 5 years of there life?

Hi Sophie, there are a few animals that change sex over the course of their life (this is called dichogamy or sequential hermaphroditism) and they do this as a response to their environment.

Wrasse are Protogynous, as you described, because it is advantageous for there to be many females to one dominant male (polygyny) in the resource rich environment in which the wrasse live (although there will often be small non-dominant males too that try sneaky spawnings). Wrasse males hold a territory containing large numbers of female fish, when the male dies the next largest fish (either male or female) will become the dominant "super male".

This system means that many more offspring can be produced by all members of the species, either as a female or a male (or both).  This a much more efficient system than in mammals where there is one male and many females, since there will be large numbers of males that do not have the opportunity to pass on their genes at all. Mammals have their sex determined by their DNA, whereas Wrasse, clownfish etc. do not - this means that mammals are unable to undergo dichogamy.