I think Periodic Cicadas must be on the list; they only live a couple of days, and they spend most of that time looking for mates.
Mastoid
The number varies by about seventeen.
stirrup
Aphrodite did not have a birth and infancy within mythology, she emerged fully grown from the sea.
Athena.
Athena sprung out of the head of Zeus fully grown.
She sprang, fully grown, out of Zeus' head.
she sprung from Zeus's head fully grown and armored after he ate her pregnant mother
Ants are insects. Practically all normally developed insects have exactly two antennae (feelers) each, except for some of their life stages, such as eggs. All fully grown ants have two antennae each, like practically all other fully grown insects. In this they differ from say, crabs and lobsters, that have four.
they are fully grown when they are 6 years old
Athena was born out of Zeus's head after Zeus ate her mother, Metis. Athena was born fully clothed in armor and fully grown.
no. a maltese is fully grown at one year of age