Nothing against Tim Russ, but I never understood why the VGR producers didn't think any of the older African-American actors they looked at were suitable for the part. There were and are lots of great actors who could've fit the bill. Imagine Roscoe Lee Browne as Tuvok, for instance. That would've been something.
I would tend to think the explanation is similar to why Matt Smith was cast as The Doctor rather than an older actor, as originally intended by Doctor Who
's producers--a combination of a really good audition on the part of Tim Russ and the desire to cast someone young enough to handle a tough shooting schedule with its share of action scenes.