A small minority (on the order of one in one million to one in ten million or so) of men and women in this world tend to suddenly manifest their supernatural heritage/psychic abilities/superpowers not when they first become adults, but instead, in their 30s (although not one specific age and with some outliers that are a bit younger or a bit older).
This manifestation happens at about this time even in people who are completely unaware of the existence of supernaturals/psychics/superpowers and have no one who knows them who is aware of these things either.
These powers remain vital for life once they manifest, but do not prevent old aged based dementia or other serious symptoms of old age on a normal human life span.
These traits are approximately as hereditary as schizophrenia/bipolar disorder are in our world.
What plausible developmental or physiological triggers could lead to this major event being triggered at this age?
Examples of similar significant transformations in mid-life in the real world in humans or in animal models would be informative.
The trigger wouldn't necessarily have to be purely physiological, it could have some relationship to some environmental trigger that often happens around this age as well. The trigger does not have to be the same for men as for women, although it could be.
A trigger could also usually happen at a younger age and then have a long dormancy period (sort of like HIV).
By way of analogy, there are lots of physiological processes/common environmental triggers that take place in puberty/adolescence/coming of age that could be triggers from hormone shifts to thinning of neurological connections in the brain to make them more efficient but less plastic (suspected in schizophrenia/bipolar) to loss of virginity.
If there are no plausible triggers with any connection to a scientific basis, please explain why you believe this is so