In AFFC and ADWD, it’s repeatedly suggested that the North is kind of tapped out on fighting age men (barring the Boltons and a few houses that held back) as a result of Robb’s army being wiped. How likely is this to be a reality, and what does that imply about the population of the North?

I don’t think it’s suggested that at all. Stannis pick up ~4000 Northmen by basically looking through the sofa cushions, Roose has ~2,000 Northmen “allies” outside of his own Frey/Bolton forces, plus you have the Dustin/Ryswell contingent who held back, plus you have Wyman Manderly’s secret army hiding on the White Knife. 

The North’s normal military strength is ~35,000 men. Robb took only 18,000 south with him, because he was in a hurry to save Ned. That leaves 17,000 men who never went to war.