  Flares, energetic eruptions on the surfaces of stars, are an unmistakable
manifestation of magnetically driven emission. Their occurrence rates and
energy distributions trace stellar characteristics such as mass and age. But
before flares can be used to constrain stellar properties, the flaring-age-mass
relation requires proper calibration. This work sets out to quantify flaring
activity of independently age-dated main sequence stars for a broad range of
spectral types using optical light curves obtained by the Kepler satellite.
Drawing from the complete K2 archive, we searched 3435 $\sim 80$ day long light
curves of 2111 open cluster members for flares using the open-source software
packages K2SC to remove instrumental and astrophysical variability from K2
light curves, and AltaiPony to search and characterize the flare candidates. We
confirmed a total of 3844 flares on high probability open cluster members with
ages from zero age main sequence (Pleiades) to 3.6 Gyr (M67). We extended the
mass range probed in the first study of this series to span from Sun-like stars
to mid-M dwarfs. We added the Hyades (690 Myr) to the sample as a comparison
cluster to Praesepe (750 Myr), the 2.6 Gyr old Ruprecht 147, and several
hundred light curves from the late K2 Campaigns in the remaining clusters. The
flare energy distribution was similar in the entire parameter space, following
a power law relation with exponent $\alpha\approx 1.84-2.39$. The flaring rates
declined with age, and declined faster for higher mass stars. We found evidence
that a rapid decline in flaring activity occurred in M1-M2 dwarfs around
Hyades/Praesepe age, when these stars spun down to rotation periods of about 10
days, while higher mass stars had already transitioned to lower flaring rates,
and lower mass stars still resided in the saturated activity regime. (abridged)
