Well, it would probably depend on the post they hold. On Voyager, Ensign Kim was the Ops Officer. He was definately young for the post, but it was his job. So, as head of Ops, he would be considered a Senior Officer aboard Voyager.
To be more specific, Kim was one of the supposedly numerous Ops Officers of the vessel when coming aboard. A couple of hours into the mission, he had become the only surviving Ops Officer, and therefore a department head by default.
Similarly, the Flight Control Department probably lost its best and brightest when Stadi was killed. Tom Paris might have been the highest-ranking surviving specialist in that field, hence another instant department head.
In these exceptional circumstances, I see no major problem in having Kim be a "senior officer" even though fresh out of the Academy. He might well have remained the highest-ranking Ops specialist throughout the seven years, too, senior to his department even if not to the other top officers aboard. It's a separate mystery why Janeway didn't promote Kim in rank to better match his prestigious position, of course...
I don’t understand why the DS9 writers ‘downgraded’ O’Brien’s status from an officer to a non-com.
Because they needed a down-to-earth character. And because TNG "Realm of Fear" specifically needed a familiar character who would be significantly lower in rank than Reg Barclay, so that he could be ordered around by said nervous wreck.
Correct me if I’m wrong but when we first see O’Brien in TNG isn’t he a Lt. Jg?
The character played by Colm Meaney wears various pips on his collar during early TNG, including one bright (Ensign) and two bright (Lieutenant). Later on, we also learn that this character is Transporter Chief O'Brien, even though he is seen in other jobs and in different uniforms in the early episodes where the writers hadn't fully invented him yet.
However, to be sure, we never quite learn that this TNG character would be a commissioned officer. There is never direct dialogue to that effect, and indeed there is direct dialogue in "Family" to say that O'Brien is a noncom, no matter what his collar shows. The writers always felt O'Brien should be a lowly "bluecollar" type of character, even if the costuming department sometimes disagreed.
We might just as well say that our imperfect TV sets fooled us, and that O'Brien's collar always had a single dark pip to denote generic NCO rank (until replaced in DS9 by a more specific Senior Chief Petty Officer symbol).
Timo Saloniemi
Wasn't O'Brien referred to as Lieutenant once, in the second season's The Icarus Factor?
Wish I could find the piece to be able to quote provenance, but I clearly remember an interview with Gene Roddenberry or someone close to him around 1988/89 stating that there was no division between Officers and Enlisted in Starfleet (TNG Starfleet, that is, there clearly was in TOS) - the Enterprise carried civilian specialists, families, and Starfleet officers, and everyone in Starfleet was an officer who'd graduated from the academy. That idea obviously went by the way in later years.