If the North had another 17000 potential recruits, why didn’t Robb call for some reinforcements ? Like when, Edmure gave Riverlords leave to return to their lands, Robb could have brought a few thousand men south to strengthen his depleted forces. Or he could have ordered them to descend on Moat Cailin from the North, while he, Greatjon & Bolton led the 3 divisions up the Causeway. That seems to have been the simpler solution, rather than the risky detour through the marshes he was planning.

There’s several factors:

  1. Bringing up reinforcements doesn’t become relevant until the very end of AGOT, when Robb’s attempt to free his father by defeating the Lannisters and forcing a prisoner exchange fails, due to Joffrey killing Ned. 
  2. The leadership remaining in the North consist of Bran (9 years old), Maester Luwin (non-combatant), and Ser Rodrik Cassel, who is a better soldier than a general. In fact, Ser Rodrik will order reinforcements (the Wild Hares) to de-mobilize, then go haring off himself to deal with Ramsay, then shuttle back to Winterfell when Theon takes Winterfell. So there’s no one in the North who’s up for the job of calling the banners. 
  3. Speaking of Ramsay, the deaths of Lord Hornwood and his heir mean that the Hornwood lands spark a crisis, whereby the remaining lords of the North are divided by their competition to win the lands of House Hornwood. When Ramsay Snow abducts Lady Hornwood, this changes from a peaceful but distracting political contest into all-out war, as the Manderlys and the Boltons go to war, and Ser Rodrik goes to war to try to defuse the conflict. 
  4. Speaking of the Ironborn, the capture of Moat Cailin, Winterfell, Deepwood Motte, and Torrhen’s Square, along with the attack on the Stony Shore, make reinforcement practically impossible because the political nerve center of the North is paralyzed (so there’s nowhere to rally the banners to) and the main land bridge by which those reinforcements would march south across is cut off. 
  5. And then, to put the cherry on the sundae, Ramsay sacks and burns Winterfell, so that as far as anyone in the North knows, there is no Stark in Winterfell, and kills Maester Luwin and Ser Rodrik, so that the not-particularly-great adult leadership that was left in the North is now completely gone. 

At what age would people in ASOIAF not try and marry widows like Donella Hornwood? She wasn’t able to have more children because of her age, but it seemed like everyone and their cousin was trying to marry her.

It’s not about child-bearing, since her children by someone else wouldn’t have a right of inheritance to her former husband’s lands. Rather, it’s about access to a widow’s “use-rights.” In many cultures, prior to the invention of life insurance or survivor’s insurance, widows had a customary right to use at least part of their former spouse’s estate for the rest of their lifetime, to ensure that they wouldn’t be left destitute.

Thus, marrying Lady Hornwood would give her suitor a claim to the Hornwood lands, at least for the duration of her lifespan. (Another sign of the Boltons’ abuse of the social contract is that Ramsay claims a permanent right to the Hornwood lands due to his forced marriage to/abduction of the widow Hornwood).