"As people begin to age, they begin to experience a lot of loss and a lot of grief," Hullinger said.
Friends may have died. Families may have moved away.
"It's a very different time now, with families spreading far across the country and the world," Hullinger said. "They may not see family members as much. That isolation can lead to depression."
Seniors may find isolation compounded if they are caregivers or if they have their own health limitations.
"That may change their abilities to do certain things in the same way they used to do things," he added.
Seniors whose careers are winding down or seniors in retirement are at particular risk. Many have good financial retirement plans. But few consider their retirement mental health, he said.
"We think a lot about retirement when we are working. We can't wait to retire," Hullinger said. "But when people's identities are so enmeshed with their jobs, when they're not at work anymore, they feel almost at a loss at what to do."