The Eldar grew in bodily form slower than Men, but in mind more swiftly. They learned to speak before they were one year old; and in the same time they learned to walk and dance, for their wills came soon to the mastery of their bodies. Nonetheless there was less difference between the two Kindreds, Elves and Men, in early youth; and a man who watched elf-children at play might well have believed that they were the children of Men, of some fair and happy people. For in their early days elf-children delighted still in the world about them, and the burden of memory was still light upon them.
This same watcher might indeed have wondered at the small limbs and stature of these children, judging their age by their skill in words and grace in motion. For at the end of the third year mortal children began to outstrip the Elves... Children of Men might reach their full height while Eldar of the same age were still in body like to mortals of no more than seven years. Not until the fiftieth year did the Eldar attain the stature and shape in which their lives would afterwards endure...

Well my Elf friends, Arwen's child seems close enough to being an Elven child, right?