Re comment: "Is PPMS rare in young people or is it somewhat common. If it isn't common then why have so many young people actually developed it. Also, are young people better able to stave off disability due to PPMS?"
1. Approximately 15% of MS'ers have PPMS.
2. The average age of onset is 39 years of age.
3. 95% of MS'ers with PPMS will have an age of onset of between 19 and 59 years; therefore it is not that uncommon to develop it at a young age.
4. On average PPMS progresses as fast, or as slow, as SPMS.
5. The younger you are the better the prognosis in terms of time it takes you to reach a specific disability milestone, for example needing a walking aid. This is not necessarily very helpful as the MS is with you for life and you will eventually get to that milestone even though it may take a bit longer.
If you are interested in reading more about the natural history of PPMS I suggest the following paper, which is free to download.
Kremenchutzky M, Rice GP, Baskerville J, Wingerchuk DM, Ebers GC. The natural history of multiple sclerosis: a geographically based study 9: observations on the progressive phase of the disease. Brain. 2006 Mar;129(Pt 3):584-94.