I feel like witches need to be immortal or atleast create a potion to become immortal. because they are lifestates of pure magic i suspect their magic would keep them alive much longer than normal sims.

Download 'Individual aging' and turn aging off for your witches. http://www.modthesims.info/download.php?t=160414 You do have to be careful not to turn it off on sims you don't mean to as it only takes one click to turn it on or off. It's an old mod but I still use it with all EP's.

or if you want a non cheat way you could get a cow plant and kill off townies and drink their essence. That gives 5 extra days. That would only suit an evil witch though.

There is a potion like that. It's called Elixir of Life... (Yeah, I know, not exclusive to witches - but it could be in your gameplay.)

There is a buyable one but your sim still needs to be in gold to use it.

I tend to think of the elixir of life as being a witchy type thing. I don't let just ordinary sims have aspiration rewards. They have to have a reason to have this rare tech!