There's a negative stat effect added to proxies' mood called "contract_age". I don't see anything in the files we can readily modify that refers to it, so I think it's in the core game engine. I'm not sure at what point it kicks in.
If that is the case, you could modify the start_walking.walk event file to check the value of the effect, and apply an equal positive effect to generic.mood. Something like this I think would work (this applies the adjustment effect for 24 hours, although I'm not sure if the negative effect is calculated by the game engine hourly or just at the start of each day):
adjustment = proxy.getEffectValue("generic.mood", "contract_age") * -1;
proxy.updateEffect("generic.mood", "age_fix", adjustment, 24);