Largely because that's when they finally have the money (and/or credit) to do it. Probably in their prime earning years, children are out of the house, bought a house well before housing costs went insane, are old enough to have savings but young enough to not be burning through it for medical reasons... They also starting driving in a time when cars were more throw-away but more affordable for the average blue collar worker so they're more likely to trade out based on some random mileage number, because the warranty expired and they are scared 'of all this modern stuff you can't work on yourself', and because they are still keeping up with the Joneses even though they'll vehemently deny it.
It's not that people in their 20s-30s don't want an expensive new truck, after all. It's just they lack the means.
Maybe Joe will pay off truck loans next.