To be eligible for juniors, skaters must be between 13 and 19 years old (or 21 for the male in pairs/dance) by the start of the season (July 1).
To be eligible for seniors, skaters must be 15 years old by the start of the season (July 1).
Juniors can move up whenever they want, as long as they are eligible for seniors. Some skaters will move up right away, others will wait until they age out, and others somewhere in the middle.
In Marjorie and Zach's case, they were 17 and 19 at the beginning of the season. They are in the overlap area where they could compete in either juniors or seniors (or both, but that's less common in dance than other disciplines because the required pattern is different). They won't age out for another two seasons, though I won't be surprised if they move up before that