The tendency of RR to convert to SP at around age 40 is something I've read many places. Wikipedia has this:
The age of onset for the primary progressive subtype is later than for the relapsing-remitting, but similar to mean age of progression between the relapsing-remitting and the secondary progressive. In both cases it is around 40 years of age.
with the research cited  being this:http://www.ncbi.nlm.nih.gov/pubmed/18970977
ms.about.com says this:
http://ms.about.com/od/multiplesclerosi ... s_spms.htm
People with later age of onset: RRMS tends to move into SPMS around age 40. This means that people who were diagnosed with RRMS at a later age tend to move more quickly into SPMS. Again, this is in untreated RRMS.
but no supporting research is listed.
I'd be surprised if it's stated as frequently as it is, to learn that there is no research supporting it, but I haven't the time to dig through google scholar, so who knows.
I am approaching 40 in a few years, so it is a matter of some concern. Age 39 - 40 is also when the brain peaks in its amount of myelin, in the healthy population, and after that, gradually declines.