Ya I'm not sure there is such a thing as too old on well stored ammo, but if there is I ought to figure it out.
Most of the ammo I've got is in deep storage mode, packed tightly into gasketed cans with dry packs, stacked up in a dry area. My ASSumption is it can sit there for decades and come out in about the same condition as it went in.
Your ASSumption is correct.
The only thing I've found with long term stored milsurp ammo ( primarily com bloc stuff) is acidic paper/ boxes used in packing. I've switched to unboxing the stuff and storing it loose in cans. No paper to attract moisture, no potential acidic interactions with brass, copper washed steel, or copper clad bullets.
I'll toss in a couple of Lot# portions of boxes so the mfgr date and Lot are available for reference.