Here is a answer for you. I googled it.
Epoxy gets its strength when a matrix is formed, which happens because "amine" (NH3) groups on each hardener molecule form strong bonds with "epoxy" groups on each resin molecule. (This part I am sure of; all the rest is a guess.)
When hardener ages, some of the NH3 bonds are broken by water and CO2 in the air. NH4 (ammonia gas) is released (thus the smell), and amine blush molecules are formed.
Now there are fewer NH3 groups to bond with the epoxy groups. Thus, the curing reaction time increases.