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).