I graduated with my bachelor's degree in 1987 and went back for a master's degree in 2000. I noticed I was not able to coast through my MS like I did my BS, and it was a real chore to do it on top of a job. But my grades were better (a 3.94 compared to a 3.74 GPA) despite harder cpursework due to better discipline and a better work ethic than I had as a college kid minoring in alcohol and women.
I work as a vocational rehabilitation counselor and about 1/2 my students are non-traditional students. There are usually some rust issues in math and English that can make it hard to get up to speed and may add a semester of remedial work. But once there, they tend to do very well with better work ethics and more life experience/common sense to keep them well grounded in class dicussions, research papers, etc.
Unlike most college kids, non traditional students usually know what they want and are both more focused and more motivated. Focus and motivation are just as important, if not more important, than intellectual ability in college.