A comeback name has to be old enough that it isn't still attached to someone's parents, but not so old that it sounds like a museum label. There's roughly a 70 to 100 year window where a name has aged out of recent association but hasn't yet become unreadable. That's where most current revivals are sitting.
Old names with new momentum
Latest recorded counts versus historic peaks in the SSA dataset.
Henry 11,412 peak, 10,406 latest
Eleanor 8,498 peak, 5,519 latest
Hazel 7,615 peak, 5,004 latest
Theodore 5,911 latest (a new high)
Josephine 8,683 peak, 2,791 latest
Arthur 10,527 peak, 1,503 latest
These aren't all the same kind of comeback. Henry barely left, so its current numbers are continuity rather than revival. Eleanor and Hazel made the full loop from antique to fashionable. Theodore is past its old peak. Arthur and Josephine are quieter, which may be exactly why they appeal to parents who want a familiar name without picking one already saturated in their friend group.
The rule of thumb: a name needs about three generations of distance before it's available again. A name from the parent generation feels too close. A name from the grandparent or great-grandparent generation can feel newly usable.
Browse more on the Comebacks page, or inspect Hazel, Eleanor, Theodore, Arthur, and Josephine.