>When wither lands aging effect, does it add to your current
>age, or reduce the maximum?

The latter.

>Are the effects proportional to your lifespan, or do
>short-lived races suffer more?

The latter. Specifically, it's always a certain range of number of years, regardless of whether your race lives to be 60 or 600.

>How do you know if you have been aged by wither?

You get a different echo about it ripping years from your life. It might even be colored a scary red, I can't remember for sure offhand.

>I heard there is an edge that gets back years lost to
>withering. If it is true, how is it called?

Not that I'm aware of.