the only way to explain the sudden and highly pronounced drop-off between 29 and 30 not only here but on the year-by-year graph linked a few days ago (from a single cycle) is that PG considers exactly 30 to be "over the hill" but not 29, and, therefore, being 30 is a very heavy marginal disadvantage versus being 29 when trying to gain acceptance to YC.

There is no way there is really that much of a difference in applicant quality and quantity at that margin: it has to be YC's selection bias.