I came up with an alien species that has the following reproductive life: 

* They hatch out of eggs, tended by their mothers, and by the end of the first year (about two Earth years long) they are ready to mate as males.
* After mating they go into a metamorphosis cave; by spring they will come out as slightly larger females.
* They are still not mature, and need a few adults to supervise them through this second year. In fall, the cool temperatures again trigger mating urges, this time they mate as females, with males who are a year younger. Then it's off to some caves where they go into a semi-hibernating state, keeping their eggs warm, hatching them, and feeding the the young through a lactation tube.
* These third-year females tend the males all year, then shepherd them to the mating arena, and on to the male-to-female metamorphosis cave--then they are finished with motherhood, and free to go to the cave where their second metamorphosis will take place, leaving them as (still somewhat small) adults.
* For this species, sex is for kids. When they need to limit their numbers, they can manipulate conditions so that males, or females, can change directly into adults without mating (though they will be smaller than normal).

My question is--is it illogical for evolution to deliver a species that has a metamorphosis AFTER breeding? Earth creatures do their breeding in the final state. Perhaps it serves no "purpose" to have a major change after one's reproduction is completed. But I can see one precedent, in human menopause; the thesis I've seen is that older women can assist their daughters in successfully raising their grandchildren, thus improving their DNA's success rate better than by having more children themselves when they're not young and spry. The second metamorphosis of the aliens could have come about as a "mistake" that was retained because it was useful. What do you think?