While it's true that the
Vorlons were one of the younger of the 'First Ones' the Shadows were actually the oldest of all races except for the race that 'Lorien' came from. This is clearly stated in the original scripts by JMS, it's not some deduction or assumption.
http://www.midwinter.com/lurk/guide/038.html
"In the Shadow of Z'ha'dum"
Delenn:
We have no other name for them. The Shadows were old when even the ancients were young. They battled one another over and over across a million years. The last Great War against the Shadows was ten thousand years ago. It was the last time the ancients walked openly among us. The Shadows were only defeated, not destroyed. A thousand years ago, the Shadows returned to their places of power, rebuilt them, and began to stretch forth their hand. Before they could strike, they were defeated by an alliance of worlds, including the Minbari and the few remaining First Ones who had not yet passed beyond the veil. When they had finished, the First Ones went away. All but one.