age_at

age_at(subjects, as_of)

Add a source’s staleness, as a value, beside each source’s known_at.

Staleness has to reach a method as a value, not as rows quietly dropped by latest_before’s maximum_age. Filtering it upstream would make a stale prediction indistinguishable from a missing one downstream.

Parameters

Name	Type	Description	Default
subjects	pd.DataFrame	One row per subject, with a _known_at column per source, as pivot_sources produces.	required
as_of	pd.Timestamp	UTC-aware reading moment; invalid values raise PanelError.	required

Returns

Name	Type	Description
	pd.DataFrame	A copy of subjects with one _age column per _known_at column, holding as_of minus known_at. NaT in, NaT out.