Genesis never said that the life that went into the ark was adults in age. And to be precise, Genesis never said that the ark carried each species, but each kind. The classification by species is but a few hundered years old.
Creationism is so anti-conservative. It involves a blind faith that is threatened by science/rational thought. It attacks what it sees as the alternative(science) yet claims that any discussion of itself is invalid because it is anti-God(CK post 92). The progressive/socialists/communists dream of Man induced Heaven on Earth/social justice/world peace is also untouchable. Like Creationism it has had its day, a complete and utter failure. The believers are out to destroy the alternative (liberal-democratic-capitalism) and any discussion of the failed Dream of Heaven on Earth(like Creationism) results in an attack as a racist, sexist, homophobe, tool of the corporations (or called atheist by Creationists.) Both are any means justifies the end/ 'can't be debated' beliefs and are scary.
So, the incredible variation in species we have now, including those that went recently extinct, is a result of 4500 years of evolution? WOW.