The problem is, evolution and epidemiology collide/coincide on a different basis than you'd think, from a self-centered humanitarian standpoint. Richard Preston made the point in The Dead Zone that really, Ebola is a terrible failure of a disease - it's most advantageous to the organism if it can spread and multiply without injuring its host badly enough to provoke shunning and quarantine. You look at the common cold, which can be caused by a startling menagerie of viruses, and you see a large cohort of successful organisms that spread ambitiously but do not impact their hosts strongly enough to cause any prevention. You look at rabies and you see an organism so virulent that all other hosts avoid the infected.
But evolution is a spontaneous process involving mutation good and bad and sometimes the benign pops up and kills the shit out of everyone. The Spanish Flu of 1918, for example. It wasn't just that it was a particularly nasty strain of influenza, it was also a bad circumstance for survival - lots of stressed out soldiers cramped in military hospitals, lots of travel with a bare modicum of quarantine procedure in place, etc.
Hantavirus cropped up and killed twelve people back in '93. It generally doesn't kill anyone - I grew up with plague and by the time I was in high school it had killed two classmates so we never learned shit about hantavirus. But apparently the Navajos had dealt with outbreaks for a hundred years or more. Compare and contrast: sweating sickness killed you fucking dead within hours and that was probably hantavirus, too.
But we don't even know that. When Preston wrote Hot Zone back in '93, we thought Marburg and Ebola were the same virus. Now we don't. We think Marburg came out of a cave. But we don't know for sure. So. Does the age of a pathogen affect our susceptibility to them? Probably yes, probably no, probably maybe. Epidemiology is a live science. Shit changes a lot.