Sometimes this kind of questions just come into my mind.
Do familiars age, grow old and die of old age?
The logical answer is that they do, but then more questions start popping out.
If a Witch lost her familiar because of old age, she'd lost many of her spells as they would be placed in her familiar.
She wouldn't be able to teach spells to a new one until she lost the first, so it doesn't seem possible to simply «transcribe» the spells to the new familiar.
Is there some oficial ruling about that?
How would you deal with it?
I always figure something that familiar lifespans were extended to match that of their spellcasters. Otherwise and elf would never bother to have a standard familiar. From the elfs perspective they'd have to constantly get a new one.
I remember having this doubt back in 3.5 when I first read the Draconomicon and I read about having dragons as familiars.
It said something that it had to be a wyrmling and could only be a wyrmling.
I don't remember the wording but whatever: if the dragon could grow up you'd only have a familiar for 5 years, if it didn't you'd be forcing the poor creature to be a baby for a lot of time.
The issue came back to my mind regarding to witch familiars, because having a «spellbook» that can age and die is a dangerous thing.
Since a familiar is a spirit in animal form, I would believe it to have a magically enhanced lifespan. It isn't an animal anymore, it's a magical beast.
There aren't any rules specifically dealing with this, however, but I would call it a safe assumption that the creature would live far longer than a normal animal of it's kind.
James Jacobs suggests that they get the longer of the lifespans of the master and the normal animal. 'Course that's JJ's opinion, not a rule, as always, and he offers no support for it.
I'm sure my owl familiar is glad to hear this ;-D