It means she's older than 40.

If she's on the wrong side, I guess she's older :) At least that's what I think. Where do you get all those phrases?

It usually means your older than 45, so its usually said on your 46th + birthdays. As you are nearer to 50 than to 40, so you're on the wrong side!

This expression is only used for someone who is older than forty. Those who are over forty understand that this is nonsense because life begins to become meaningful after forty or fifty.

It means she is either TOO old, or TOO young. "On the wrong side" means her age is undesirable, unwanted, not good. If she wanted to have a baby, you could say she is "on the wrong side of 40", because after 40, complications are more likely. If she wanted to be a model, you could also say she is "on the wrong side of 40", because she is no longer young and probably not as pretty or fit as a younger person. It's also possible she's too young for something, like being a President maybe, because most candidates for presidency are much older, so she would also be "on the wrong side of 40".