I browse around in the medical literature and have been noticing that lately there's been discussion of evidence that the MS drugs aren't effective in persons over a certain age--60 or 65 or thereabouts. The question is usually whether a doctor should recommend discontinuing them at that point.
Some people are finding that they're no longer covered for Tysabri, for instance, when they reach age 65. This is probably a determination aimed primarily at saving money, but there may be medical reasons to support it.
I have backed away from any neuro suggestions that I resume MS drugs mainly because of my age. The MS drugs are powerful, and they involve the immune system--and they have their risks.
It's fairly well established that as we get older, drugs affect us quite differently. They are either more or less potent than in younger people, and the side effects can be quite different. It's for those reasons that I hesitate to try any drug aside from the ones I've been taking for years, and even with those, I'm trying to cut down the dosages wherever I can, just to be on the safe side.
They do know that older people respond differently to drugs. What they don't seem to know much about is just what goes on and with which drugs.
"Less is more" sometimes.