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 Cockapoo, watch their ears, their eyes and the way they move. A Cockapoo who shakes their head a lot, skips a step on a back leg, licks their paws for hours or gets nervous going out after dark is telling you something. From four years, a dog who slows down and gains weight on the same food can have an underactive thyroid. In an older Cockapoo, pacing or staring at night can be the start of Cognitive Dysfunction (dementia in older pets). Bloat (GDV, a twisted stomach) can happen in any dog, though it is most common in large, deep-chested breeds. 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 mood or age. Tell your vet anyway, because early makes a difference.
They'll keep sniffing out their dinner. If they stop hunting for it, tell your vet.