Just how little mass is required to instigate the nuclear fusion process motivates much contemporary research. If there's too little mass, the central pressure never climbs high enough to trigger fusion. The act of formation provides enough heat for the youngest of these objects to produce visible light, just like an iron bar recently removed from the forge. We see them in the process of cooling. Spectroscopy provides the critical diagnosis.
Red dwarfs are a stopover on a continuing journey of discovery.
We used to think they were the coolest stars, but no more. But they continue to have value, since binary stars containing M dwarfs are the hunting grounds for brown dwarfs. Whatever processes form stars, they seem to favor low masses, since there are more M dwarfs than any other kind of star. Sky surveys toil to answer this question. The journey continues.