He's not a student, he's in his 20s at the start of the film. So growing up during the trilogy is not a concern.
Also keep in mind that while the students were unknowns in the films, almost every single adult was a famous British actor/actress. Richard Harris, Ralph Fiennes, Helena Bonham-Carter, Alan Rickman, Maggie Smith, Robbie Coltrane, Michael Gambon, Kenneth Brannagh, David Thewlis, and Julie Walters were all well-known. It wouldn't be out of character of the series to cast a well-known actor (and I think they will). I personally would like Matt Smith. He has the youth and would fit the tone of the universe.