I don't know how common it is, but I do know that some neuro's feel confident telling patients that they have a mild case, and that they don't expect it to progress quickly. I'd be surprised if anyone ever produced any data
about their predictions, though!
My neuro has called my MS one of the mildest cases he's seen, apparently based on my sx (>90% paresthesia) and handful of lesions (no Optic Neuritis). I just saw him in May and he wasn't at all surprised that my SX haven't changed (except for intensity) in about two years.
I'm 56, but I doubt age makes much of a difference...