I’ll be shocked if anyone gives you a great answer as we don’t have the greatest experience with older pigs in America. Here in the States we turn our pigs over rather fast in comparison to in the UK or Canada. In Canada pigs are kept breeding longer because there are so few LB in the country.
In the UK, they are very selective in what is a breeder and what is a feeder pig. MANY are feeders with few becoming breeders, mainly to replace their own parent on the farm. You can look through the registries and you will find sows as old as 12 years birthing litters while this is not the norm. I have seen 6-8 years as the normal with 2 litters produced per year. While it doesn’t happen often a sow can breed past her 8th year but they must be kept fit to do so. Litters in the UK are notified at birth and then only breeders are registered. Here we get a litter certificate and every animal is registerable. Breeders should use judgement and a discerning eye but too often that is not the case and the entire litter ends up being registered/sold as breeders.
Look to number and health of piglets as well as the health of the mother in the three phases of motherhood: pregnancy, nursing, and while open. If she is losing condition during any of these three she may need more exercise, better quality water/food, or better shelter. Her calories should be spent on her fitness and the health of her babies while in utero/while nursing.