Yes, M-stars do evolve, just not in the classical sense that we are used to with solar-type stars...
The image below illustrates the NUCLEAR evolution of four types of star: F-, G-, K- and M-. As you can see, the M-star's evolution (bottom red line) is just about negligible. This means that if you were to age an M-star based on its luminosity, your error bar would go roughly from its Main Sequence birth to its death. This would give you an error on the order of tens of billions of years. Such a large error is simply inadequate.
You are, however, still left with the unavoidable fact that age is one of the most fundamental stellar properties. With M-stars making up >75% of the local stellar neighborhood (and it is assumed they are just as common throughout the Universe), being able to assign them reliable ages would also have a far-reaching impact on many fields of Astronomy - from Stellar Evolution (naturally) and Structure to Planetary Studies all the way to Galactic Structure & Evolution. If the Milky Way is comprised of so many M-stars, being able to age them all will allow you to determine when various parts of the Milky Way underwent star formation.
Planetary Studies can benefit from M-star ages in a very important way. More recent (and future) ground-based exoplanet surveys such as the MEarth project (http://fr.arxiv.org/abs/0807.1316) and the PRVS program (Operational in 2011 - http://adsabs.harvard.edu/abs/2007AAS...211.1702J) will be specifically targeting M-stars. Also, the more long-running exoplanet surveys, such as the California & Carnegie Planet Search (CCPS - http://exoplanets.org/) and the High Accuracy Radial velocity Planetary Search project (HARPS - http://obswww.unige.ch/Instruments/harps/Welcome.html) are observing an increasing number of M-stars in their continuing and very successful planetary surveys. If you can determine the age of a star, then you know the ages of the planets orbiting that star. With that knowledge in hand, you can being to analyze the stability of the planetary system. Also, if the planet(s) is (are) known or suspected to be habitable, you can also calculate how much time life may have had to form.
So, back to the point brought up in the second paragraph: M-stars can not be reliable 'aged' through standard methods which rely on the nuclear evolution of a star. Therefore, is a nuclear-related property (such as luminosity) can not work, we must find a property that does. That property is MAGNETIC ACTIVITY.