You must live in B.C., I believe that they are the only province that does that. Certainly more people over 65 get medical exemptions than the general population. Also note that a jury is supposed to be of ones peers, so if it was entirely people over age 65 that would be a very biased view.
"A jury of your peers" doesn't mean people of your specific demographic profile. A woman doesn't get an all female jury, a Muslim doesn't get an all-Muslim jury, or anything like that. A career criminal doesn't get a jury of other career criminals. Your peers are your equals-- other free citizens of Canada.
-k