Pure copper and gilding metal (which is a solid solution of zinc in copper) don't become harder or more brittle simply as a result of age.
Age-hardening is a well-studied phenomenon. It occurs when a second phase is dissolved (by heating) and then, with time (and usually temperature) precipitates out of solution to pin dislocations in the crystal lattice, thereby reducing the ability of molecules to slip, which is seen on the macro level as increased hardness.
Work hardening is where cold working increases the density of dislocations, which again reduces the ability of molecules in the crystal lattice to slip.
Age hardening doesn't occur in pure copper or gilding metal because there's no metastable solution of a second phase to precipitate.
What can cause changes in physical properties with time is degradation, such as corrosion, but if your bullets have been well stored and show no obvious signs of corrosion they should be fine.