The Hollow Knight: Silksong Passing of the Age quest comes from a riddle altar in Putrefied Docks. It’s at the lower-right corner of the zone just past Fleatopia–i.e. the location of the Flea Caravan after you’ve rescued at least 25 Lost Fleas.

Climb onto the ledge and look above you. Then, use your Silksoar maneuver to reach the hidden nook. The riddle has the following clues:

Under chapel fallen to the monarch’s might,

Over camp collapsed to the black thread plight,

Scorched field on edge, shells black and burned,

Up towers spinning, near the surgeon spurned,

Cage of one who raged against the silken lie,

Fell heart of frost who soars on high,

Past cradle’s peak, last test of fitness,

That the age may pass, this shroom its witness.

Note: The specific locations that you visit should have blue mushrooms on the ground. Stand near these and play your Needolin to make Mister Mushroom appear. Talk to him, then make your way to the next area in the sequence.