Life expectancy for those with MS, from what I have read, is a complex issue and a changing one. Jacqueline du Pre the famous cellist died at 42 about 15 years after diagnosis, I read that the mother of J K Rowling (Harry Potter author) died in her mid 40s. But many family members have told me of Mrs X who is 80 and has ms etc etc. My neuro told me that those with MS live a 'near normal life span'. It seems to depend on your sex, what type of ms you have and how aggressive it is and what functions it disables. Also, just because you have ms, you must have pretty much the same chance as getting heart disease, cancer etc as if you didn't get ms. The other complication is suicide which is 7 times higher among the ms population than the general population (mainly young men). Every case will be different and as medicine progresses the life span of those with ms should improve.
The real issue is not really life span but quality of life. Some ms sufferers experience paralysis from the neck down (rare but it happens). I think I would swap 10 years of being mildly disabled than 20 years in a much worse situation. But these are of course personal decisions and each person with this disease will have their own view.
Hopefully, with more treatments coming on stream and better care etc the vast majority of those with ms can enjoy a near normal lifespan and a reasonable quality of life.
All the best