Due to his age, Nolan will probably always be the oldest rookie at every stage of his career at the LAPD, assuming his LAPD career can continue after however the Season 3 premiere handles the cliffhanger!
The framing of John Nolan that did end up closing Season 2 may mean that The Rookie won’t have to deal with Nolan being promoted too far out of rookie status any time soon, although the showrunner does have a plan to address that potential problem.