@abetterjulie@wandering.shop The (very condensed) religious answer to this is that Buddhism generally conceptualizes the linear flow of time as part of the illusion of the material world; the state of Enlightenment frees someone from it in the same way as it frees someone from the physical constraints of space.

The Dalai Lama is a tulku; basically a fragment of the Buddha/someone who has previously achieved enlightenment that is sent back into the material world to assist others in achieving the same. Because the "enlightened self" of the tulku transcends linear time as we understand it, they/it could theoretically manifest however and whenever it chose. If reincarnations have previously occurred in a linear way that's because that was the most effective way to present to material society, not because it "had" to do it that way.

(There's also more in here about how Buddhism in general and Tibetan Buddhism specifically conceptualizes souls and the self. The tl;dr is that it's not as simple as the Christian concept of "one person = one soul," souls aren't fixed in nature, and the idea of an individual personal soul at all is part of the illusion of the material world that doesn't survive enlightenment. Everyone's "soul" is Buddha-natured, and that realization could occur at any time; that's what enlightenment fundamentally is.)

All that being said, the pragmatic political reason is that it's probably easier for an adult lama to avoid persecution by the CCP than a child (ditto with the current Dalai Lama pretty strongly hinting his reincarnation won't necessarily be Tibetan/Chinese). So. Yanno. It is what it is.