Perhaps Berman/Paramount had a hope that this crew would make it to the big screen, so they wanted younger-ish actors?
No, IIRC, Tim Russ auditioned for TNG and the TOS movies a lot
, and was a favourite with the casting people. He managed to score a few small roles with them prior to Tuvok. He was very nearly Geordi. Although Tuvok's description was for an older actor, Russ was able to audition and managed to impress the right people at the right time.