Fourthlings have a short life expectancy and/or age more rapidly than other races.Think about it. Most of the main and supporting characters get married and start having kids really early. This is because fourthlings need to leave enough time to see their kids grow up before they die. Quint is in his thirties in the Twig trilogy, but is presented as much older; this troper would never have guessed he was so young before she saw the timeline. Then there's Rook's portrait in The Immortals: it was painted 'early in the Third Age of Flight' and Rook was in his thirties when the Third Age began, but he's gone mostly grey. Of course, there are a lot of old professors running (hobbling?) around, which leads to my next WMG...
Exposure to stormphrax increases a fourthling's lifespan.How many old fourthlings do we see who aren't sky scholars? Stormphrax is kept in the heart of Sanctaphrax, and its life-lengthening effects filter through the whole of the rock, thereby affecting everyone who lives on it. It's more prominent in fourthlings because they don't live as long (see above WMG). It also explains how Twig lived so long: he was actually in the Twilight Woods in the middle of a Great Storm, and was exposed to stormphrax again when he brought Screed Toe-Taker's chest back to Sanctaphrax, the concentrated dose doing the same thing as smaller exposure over a longer period of time. It also explains how come there were still so many old professors in the Rook era - most of the earth scholars were once sky scholars. And Philius Embertine of the Quint trilogy was, I believe, one of the oldest characters in the whole series; he went on two successful stormchasing voyages.
Keris lied to everyone in the Free Glades about when her father left.This could explain the two contradictory accounts of just when Twig left Keris to return to searching for his crew. In reality, he left when Keris was three years old, as in The Slaughterer's Quest, but when Keris reached the Free Glades, she found people practically hero worshipping her father and didn't want to make anyone think badly of him by revealing the Parental Abandonment.
At least part of the Gloamglozer's curse was that heirs of the Verginix line would never grow up with their parents.We're never really told what the curse laid on Quint is and the Gloamglozer seems like a character who would want to make Quint as miserable as possible. After Quint, the Verginix line rarely has a parent and child together for long - Quint abandons Twig in the Deepwoods, Twig leaves Keris in the Slaughterer village when she's a toddler to search for his crew, Keris is killed by slavers and Rook is taken in by the Librarians... Granted, Rook apparently doesn't die while his children are young - the portrait of him in The Immortals shows him as middle-aged - but perhaps the curse was fading after three generations. It seems to pop up again every so often, too - Nate's father dies before he reaches adulthood, too.