The Falmer are not Descended from the Aldmer

The Aldmer are claimed to be the original elven race, from which all others are descended. But this is not true. The Falmer were just as young a race as the Aldmer, they were also the first.

To make up for his error, Auri-El led the original Aldmer against the armies of Lorkhan in mythic times, vanquishing him and establishing the first kingdoms of the Altmer, Altmora and Old Ehlnofey. Auriel's Bow and Auriel's Shield were allegedly created by Anuiel to be used by Auri-El against Lorkhan's hordes. Though Altmora was retaken by men and Old Ehlnofey was sundered, Auri-El's greatest knight, Trinimac (who, in some places, was even more popular than Auri-El), succeeded in defeating Lorkhan, and Auri-El hid his Heart by attaching it to an arrow and sending it far into the sea. Later, Auri-El ascended to heaven in full observance of his followers so that they might learn the steps needed to escape the mortal plane.

This is from the lore page of Au-riel on Uesp. First off, we need to consider this. This article is incorrect, in the sense that the Ehlnofey Wars, which this writing speaks of, took place before Men, Mer, and Betmer even existed. Here's some writing on convention from the Elder Scrolls wiki, which UESP also describes on the Dawn Era page:

The Convention was an event during the Dawn Era that took place on the Adamantine Tower. Also known as the Zero Stone, Convention also serves as the Stone for said tower.

Upon the departure of the et'Ada after the creation on Mundus, Akatosh arranged a meeting of the remaining Aedra outside aurbic time. The purpose of this meeting was to decide Lorkhan's punishment for his involvement in the creation of the mortal plane.

The outcome of the Convention was to separate Lorkhan from his divine heart. Trinimac tore out Lorkhan's heart and Auri-El fastened it to an arrow and shot it into the sea where Red Mountain eventually formed.

The Heart of Lorkhan granted Mundus with a special kind of divinity and allowed Auri-El and the remaining et'Ada to leave without it becoming unraveled. The et'Ada who remained either died or eventually degenerated into the Ehlnofey, the progenitors of man and mer.

The ehlnofey wars preceded Convention. Convention was where the fate of Lorkhan's heart was decided and what would happen to the remaining original spirits on Nirn. The ones that stayed eventually evolved to become the Man, Mer, and Betmer. There were cultural differences within the Ehlnofey: The old and the wandering. But that was it. Elf and Man held no meaning then. However, I recognise that both the Ehlnofey Wars and Convention spoke of tearing out Lorkhan's heart. That's why I personally believe Convention was the end of the Ehlnofey war. And the fate of Lorkhan's heart was decided, not Lorkhan, who had already been killed.

I'll now turn your attention to the first source, as it claims Au-riel founded the Kingdoms of ALTMORA and Old Ehlnofey. Altmora. Atmora. The land to the North. So, just to make it clear, Old Ehlnofey (either Aldmeris or Summerset Isles or both), and Altmora, are just as old. We know Altmora was taken over by the Wandering Ehlnofey, so where did the elves go? I think they went to Skyrim. There, they would've eventually gone on to become the Falmer.

TL;DR The Falmer and Aldmer are just as old as each other, for the Kingdoms in which we believe both originated were founded at the same point in time, before Elves even existed.