What do spectra tell us about age (that is not encased in [Fe/H], [a/Fe])?

just a note to be brought up with Melissa, to commit to memory until she's back from Michigan:

Basis: Cannon trained with 5 labels, with ages from Kepler; with the Cannon, we get "age" recovered as a parameter with +-2 Gyrs. BUT: to which extent is the age just a corollary of [a/Fe] and [Fe/H]?

[FromMarie's work, we KNOW that there's more information ...]

Plan to find out:
Application of the age label (as is) to the DR12  RC sample, yields <age>([Fe/H],[a/Fe]);

let's define   D(age)_Kepler  == age_Kepler - <age>([Fe/H],[a/Fe])  and
                    D(age)_Cannon  == age_Cannon - <age>([Fe/H],[a/Fe])
and plot

                  D(age)_Kepler vs  D(age)_Cannon  (as opposed to age_Kepler vs age_Cannon),
i.e. we look at a version of the age vs age plot, compressed around the mean age at a given abundance.

If the Cannon teaches us nothing new about ages beyond that which could be known from
[Fe/H] and [a/Fe], we should get a scatter plot.

HWR's conjecture: the extent along the 1-to-1 line will be ~2 times the scatter..; so, there is more in the spectra...