From https://www.tng-project.org/data/docs/specifications/, I find the definition of GFM_StellarFormationTime is "The exact time (given as the scalefactor) when this star was formed".
I'm confused by this definition. How to convert GFM_StellarFormationTime to cosmic time?
It is the scale factor a. To convert to a "lookback time" or "age of the universe" (e.g. in Gyr), you need to do the relevant cosmological calculations, e.g. as implemented in astropy.cosmology.age().
Yeah, that makes sense. Thanks, Dylan!