I'm not a metallurgist, so take what I say with a huge grain of salt, but this is my take on it:
Generally, no; age shouldn't affect the ductility of steel....but...(and there's going to be a lot of buts)...it depends mostly on the alloy, the environment that its been exposed to, the geometry of the component (stress risers), and the stresses that it's had to endure. Mild steel will certainly fatigue and crack if exposed to cyclic stresses beyond an endurance limit; most guns operate with stress levels well under that. Some alloys are susceptible to intergranular stress corrosion cracking that is occasionally mistaken for fatigue when exposed to certain chemicals (chlorides and fluorides especially). Higher levels of carbon in steel tends to make it stronger, but also more brittle.
Me, I've got an old Springfield 1903 that I will not shoot because its receiver and/or bolt may have been improperly heat treated during original manufacture. Some things aren't worth the risk of ownership and that Springfield is something that eventually I'm going to have to safely dispose of.
Anyways, cast fabrications also tend to be brittle; forged parts are always stronger than cast, yet typically are more ductile. Best advice regarding antique gun metal is to read up on what you've got (or are contemplating acquiring) for metallurgical issues; and do not trust what you're not sure of either by proof testing or by anecdotal evidence (i.e. there's still a lot of old S&W's and Colt's around that haven't come apart with normal pressure loads).
As for that old 1903, I've heard that only 1 (of hundreds of thousands) early 1903 Springfield failed catastrophically due to the heat treatment issue (bolt apparently went through somebody's skull), but that's enough for me to make it into a permanent wall hanger. I'm pretty sure that uncle Sam 86'd a bunch of them as well. -S2
