...it strains credibility to have Sela be 21 years old.
Given the lack of information, clues, hints or suggestions by canon regarding Romulan Society it doesn't.
BoT suggests that good connections outweigh age, rank and accomplishment (the whole scene is somewhat reminiscent of the situation aboard the HMS Defiant
in "Damn the Defiant", where a younger officer poses a threat to an accomplished captain).
If anything, "Redemption II" re-emphasized that in the Romulan military you can climb the career ladder faster if you know the right people.
That's what I'd call continuity, not implausibility.