Other ways of determining age for M stars

M stars evolve quite slowly once they reach the main sequence. it seems that, past a certain limit, determining age using rotation is impossible since rotation is non-detectable.

For solar-type stars, stellar winds colliding with the ISM results in a pocket of hot hydrogen gas surrounding the star. This gas can be measured using absorption lines. For M stars, the same may hold true, but whether the absorption lines can be seen above the noise for these dim targets is another matter currently under investigation...