Epochful

First, an habitually good child sometimes has a saturnalia of defiance and disobedience; a series of insubordinate acts are suddenly committed which really mark the first sudden epochful and belated birth of the instinct of independence and self-regulation, on which his future manliness will depend.

Source: wiktionary

This makes the end of the century epochful and momentous.

Source: wiktionary

Kaes and Vulpius are in impressive agreement that the age of the later teens is epochful for the development of the middle layer, which then begins a new and prolonged period of growth.

Source: wiktionary