Theory #2: Ben's Ghost Is Related to the Sparrow Academy
No one is entirely sure how time does or does not work on The Umbrella Academy, that much is clear. The Hargreeves siblings have created a number of alternate timelines and it's a miracle they've managed to overcome two separate apocalypses. The idea that Ben's ghost age is tied to the Sparrow Academy suggests that it was always the group's destiny to go to 1960s Dallas and complete the events of season two.