... how is relative seniority determined?

Age? SSAN? Class standing?

Seniority within each class is determined by your age. Oldest is most senior, youngest is most junior.

What are the ages in current classes, range wise?

Everything between 23 and 55.

classes are so small it really doesnt matter. Your class might have 8 people but we're trying to hire 300 in 2018. best you can do is try to get an earlier class date

Seriously? A 300 pilot annual turnover in a company that employs 670ish pilots? That's around 45%. Are that many being hired away by the majors or are they leaving for other reasons and if so, why and where are they going?

It would seem like this would take almost everyone upgrading to captain at 1000 hours SIC just to keep the aircraft flying. How many actual block hours are FOs, especially junior FOs, flying in a month?

At Horizon many newbie E-175 FOs are mostly sitting on their butts while at Skywest they are on reserve forever, then have to switch to a CRJ out of ORD to upgrade.

Why the huge throughput at Compass, a company that no longer has a flow to the majors?

Delta is hiring our guys like crazy. Lost 11 captains to and 3 FOs to Delta this month alone. FOs are blocking 80-90+ a month.

Plus, up until about mid 2015, Compass was the only regional in the industry to require a 4 year college degree. Sure, there's a couple of guys who got in without it, but the vast majority have it. And like it or not, that seems to be what the legacies want (plus that wing-mounted engine heavy widebody 175 time).

Couple that with a (relatively) new company that everyone has either had the opportunity to flow or been in the industry only at a time when people are hiring like crazy, and we have almost no lifers, so everyone and their mom is trying to move on.

None older that 55?