Some of this is not a training problem, and treating it like one wastes time your dog may not have. If they are distressed when you leave, if they growl or guard their food, their toys or you, if they are frightened on the lead, or if anything about them changes suddenly at any age, contact your vet the same day. A tip from a website, ours included, is not the answer. If it cannot wait, contact your nearest emergency vet now.
With a Golden Retriever, watch their energy and their skin. A Golden who is suddenly quiet, goes off their food, or has a lump that was not there last week is telling you something. So is one who licks their paws for hours, shakes their head after every walk, or gains weight without eating more. In an older Golden, pacing at night can be the start of cognitive decline. Bloat (GDV, a twisted stomach) is most common in large, deep-chested dogs, so skip walks and hard play shortly before and after meals. A swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling is an emergency: contact your vet now. It is easy to put the rest down to age. Tell your vet anyway, because early makes a difference.
They'll keep carrying that ball. If they drop it and won't pick it up again, tell your vet.