The popular narrative is that Children of Asurian are limited in numbers and have stagnating birth rates, yet somehow there is always an abundance of elves to enlist in your armies. Moreover this has no effect on economic activity of your cities, although it is supposed to shrink your active workforce.
This means, that high elves have problems with census, taxation and employment , not demographics.
Ulthuan issues no passports - why would they: if you live on the Donut and have pointy ears, then you are most certainly a Child of Asurian. However, this makes it harder to keep track of real population numbers.
High Elves don‘t really retire like humans do: they are more likely to die a violent death or just give up on life. As a result all the stable jobs in Ulthuan are occupied by seniors forever. With no economic prospects the younger generations end up in gig economy: food delivery, chariot taxi, lion grooming, etc. They pay no taxes and are invisible in statistic. They are the first to enlist in armies, and the first to rebel, when public order reaches -100.
Nobility and top military brass couldn‘t be more happy seemingly endless stream of recruits ready die in endless wars in Arabia or jungles of Lustria. However, it is desperation, not patriotism, that drives these soldiers.