If the OP has the skull I can steer him to a site that will show images of teeth from hogs of known age. Unlike many other animals, the aging of hogs is done by counting molers... they add molers as they age and there jaw length increases.
There have been many 5-6+ year old feral hogs killed.
They can have litters of up to 8 but 6 is more common. The most common numbers of piglets I see runnin' with a sow is 4 with 3 coming in a close second.
The piglets can and do readily nurse from sows that are not their mother. Orphaned sucklers will be taken in by other sows of the herd.
Yes they are a pain in the butt to those who know their eco and $$$ damages. Fun pita for those who hunt them too.