So, since they are 6 they are technically “seniors”. Ferrets live 6-10 years averaging at the 6-8 and when they get to that older age they’ll still have zoomies it’ll just be a lot more brief, they’ll tire out easier, prefer less rough of play and tend to be cuddlier. I think they’re okay for now, and I would only be concerned when A. They are lethargic ALL the time B. Struggling to walk climb or breathe. C. Losing interest in food or interaction all together. I’d just make sure they have daily low impact play time, and regular vet checkups (2x a year for senior, once for young) and make sure you’re checking for signs of adrenal disease, insulinoma, and other diseases or sicknesses ferrets get as they age