Obviously a grasshopper feels pain as well -- it certainly reacts as though it does. If we are talking about the ability to consciously perceive pain and reflect on that perception, we normally restrict those types of capabilities (at least in our cognitive models) to organisms with well-developed higher brain function. Humans do not fully acquire their higher brain function until 12-24 months of age, something there is both direct physiological evidence of (undeveloped brain tissue) and indirect evidence of (lack of certain types of high-level cognitive capabilities).
Also what does "fully developed" mean? A baby's hand is a hand, but it obviously is less developed than an adult's. Does this mean that it is not a hand?
A human is born with little more than a functioning brain stem, pound for pound, maybe something like a lizard's central nervous system. The life support system, in other words. Unlike a baby's hand, which is generally functional but small, the baby's head only holds scaffolding upon which the brain tissue will grow after birth. This is absolutely necessary, since a baby's head already pushes the limits of what can reasonably fit down the birth canal. After birth, the higher brain rapidly starts to grow and assemble itself on the pre-natal scaffolding. As the brain tissue assembles itself, high-level cognitive function starts to be expressed.
There is tons of very solid evidence that the scaffolding a baby is born with cannot support cognitive function. The most poignant being that there are a number of neurological diseases in adult humans that will effectively revert the brain to something similar to that scaffolding state, all of which are marked by severe cognitive degeneration. Humans that end up in a state like a baby is born in lose all direct and indirect signs of higher cognitive function.
So the short answer is, a baby is born with many organs that are merely small versions of the adult one. However, the baby is only born with the scaffolding for a high functioning brain plus a basic animal life support system to keep the heart beating. This is actually a really neat trick -- it allows humans to have a ridiculously large brain that otherwise would not be supportable via normal reproductive processes.
Whether or not there is cognition does not matter when
discussing pain. It most certainly does not matter when
discussing the pain of abortion. As you point out, the
esoteric jargonistic definition of "perception of pain"
wouldn't even apply until a child is 2 - it's probably more
like 3 - years old.
Nevertheless, you're basing your discussion on old
information. New information is now available, through
functional scans on premies. We know that there is cortical
processing of the pain.
http://www.medicalnewstoday.com/medicalnews.php?newsid=41011
http://www.jneurosci.org/cgi/content/abstract/26/14/3662?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=pain&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT
(I also contrasted the old with the new on my blog last week.
http://www.lifeethics.org/www.lifeethics.org/2006/04/science-confirms-true-pain-in-25-week.html)