[Raynor]

It is mostly found in the Later Quenta Silmarillion:

Originally Posted by AElfwine's Preamble, Of the laws and customs among the eldar pertaining to marriage and other matters related thereto: together with the statute of Finwe and Miriel and the debate of the Valar at its making.

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 to 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.
...
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, and for some a hundred years would pass before they were full-grown.