If it is indeed simply the effects of aging, you can help reduce your dog's anxiety by
keeping floors free up clutter, taking more frequent short
walks or playing games or food puzzles to increase his mental stimulation, allow him extra space away from
strangers or stimulation when in public,
keeping a consistent routine so he knows what to expect during the day, and continuing to work with separation training for when you're away (or asleep!).
Your dog will learn how to greet
strangers with impeccable manners,
walk calmly with you through crowds and other distracting situations,
keep her calm around other canines, and work at a distance.