  The ages of stars have historically relied on isochrone fitting of
standardised grids of models. While these stellar models have provided key
constraints on observational samples of massive stars, they inherit many
systematic uncertainties, mainly in the internal mixing mechanisms applied
throughout the grid, fundamentally undermining the isochrone method. In this
work, we utilise the M-L plane of Higgins & Vink as a method of determining
stellar age, with mixing-corrected models applying a calibrated core
overshooting alpha_ov and rotation rate to fit the observational data. We
provide multiple test-beds to showcase our new method, while also providing
comparisons to the commonly-used isochrone method, highlighting the dominant
systematic errors. We reproduce the evolution of individual O stars, and
analyse the wider sample of O and B supergiants from the VLT-FLAMES Tarantula
Survey, providing dedicated models with estimates for alpha_ov,
Omega/Omega_crit, and ultimately stellar ages. The M-L plane highlights a large
discrepancy in the spectroscopic masses of the O supergiant sample. Furthermore
the M-L plane also demonstrates that the evolutionary masses of the B
supergiant sample are inappropriate. Finally, we utilise detached eclipsing
binaries, VFTS 642 and VFTS 500, and present their ages resulting from their
precise dynamical masses, offering an opportunity to constrain their interior
mixing. For the near-TAMS system, VFTS 500, we find that both components
require a large amount of core overshooting (alpha_ov ~ 0.5), implying an
extended main-sequence width. We hence infer that the vast majority of B
supergiants are still burning hydrogen in their cores.
