I've been using MC-TEA for something like 3 years. It darkens over time, but it does so evenly, throughout the solution. If the darkening was due to aerial oxidation, I'd expect it to be strongest at the surface, given that TEA is so viscous. That's what happened with a solution I made of phenidone in propylene glycol - it went red at the surface, but stayed clear below that.
I also haven't noticed any real change in the activity of MC-TEA over time, despite the darkening, though I haven't tested this very carefully.
Put supposing activity over time is nearly intact, that plus the even nature of the darkening leads me to wonder if it is due to some slow reaction in the solution other than aerial oxidation. Perhaps it's a reaction between impurities.