[pfdietz]

M dwarf stars, being fully convective (at least, the ones lighter than 0.35 Msun), cycle their entire material content through their cores. This is unlike the Sun, where the core is effectively isolated from overlying layers, and will run out of hydrogen while those outer layers still contain a great deal of it.

As a result, and due to their low luminosity, M dwarf stars can go on burning hydrogen for a very long time, perhaps as long as 12 trillion years for a 0.1 Msun star, much longer than the universe has existed so far.