There really isn't a great biological trigger - puberty is too young, end of puberty a bit too high (some changes in some people lasting into the 20s, not ideal if 18 has to be the cutoff - not to mention some medical conditions might apply).
But, you might find environmental triggers. So, what do adults do that under-18s don't? ... well, nothing. All the age-related activities are restrictions, and therefore optional, and adults who did opt out will survive, skewing your population. On the other hand, what do under-18s do that adult's don't? Hah - maybe school works.
If school is compulsory (like in our society), then 18 would be about the age of graduation. I recall we had a daily fluoride rinse program in our school - not strictly relevant, but there may be some other health-based program running at the right time that kids had to attend, that adults mostly were not bothering with. Maybe vitamins? Some kind of exposure from school buildings or chemistry lessons? Something that happened when the whole apocalypse got started - like barricading themselves in and avoiding the whatever? It doesn't have to be much, just enough to make it survivable.
Of course, this wouldn't be a perfect measure. There would be those younger than 18 who died because they had graduated, or dropped out, or were absent at the wrong times. There might be a few older than 18 who survived - maybe late graduating, or happened to be doing the health-whatever. They could be few enough to not be influential on your society, or perhaps they were few enough that they did wrestle with the kids for control...and lost, but they would be there.
Another option would be to tweak your society slightly, to make some event compulsory at or after 18. Maybe a citizen oath or registration or health check or something, that they have one year from their 18th birthday to accomplish - and some exposure or side effect from whatever happens during that is what makes the adults susceptible. You would loose about half of your 18's, since some would have done it and some not (or your could just say 19, I guess). You might also have a few off-the-grid types who resisted, but again the number could be small and for whatever reason not influential. Or maybe have all adults required to be present in their local coming-to-adult citizenship meeting each year (or season, etc) to affect everyone at once. Again, a few adults might be missed, but they don't have to be enough to be influential.
Or to go in a completely different direction
A lot of illnesses may have different strains or adaptions going on, and we might end up with a population exposure difference. One option might be that, if a variation of the illness (or a related one, or even a different one) was sweeping the schools just before the apocalypse-illness got started, maybe they sent all the kids they could into communal quarantine - especially once they realized kids were surviving where adults died, and maybe they didn't know it was strains rather than age-related protection.
A fair chunk of post-pubescent kids might survive just from not being exposed to the adult version, and/or getting protection from catching a lesser strain (like cowpox vs smallpox) - which showed up in schools and spread like mad. Maybe with a plot point that adults didn't catch that lesser strain because of some prior vaccination or medication or something (since phased out), which the apocalypse-virus was just different enough to slip by. Maybe it was a few quarantines of teens for an unrelated illness, with prepubescent kids surviving the apocalypse-illness due to age-related expression, with only whatever teens were in quarantine surviving due to not catching it, but afterwards taking leadership roles (and generally being visible) because the littler kids needed them.