When you say your adult starting age was 41 and it changed to 40, do you mean that adults have 41 days in that stage before becoming elders, or 41 days in all previous stages before becoming adults? If it is 41 days as adults before elderhood, then the change to 40 was probably the game aging the Sim one day, which should happen every day at 6PM unless you use another mod or cheat to alter it.
Seasons are always five days (unless you have a mod that alters season length) and the seasonal progression is completely different from the aging progression - any mod that affects one shouldn't affect the other, unless the mod changes both.
Basically, your game's probably working just fine. Your sim probably aged at 6PM on the last day of fall, then the season changed to winter - it doesn't sound like anything's messed up.
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