Dorukan was alive until very shortly before the events of the comic, and when he died, it wasn’t even from old age. And he’s human, the shortest-lived of the major PC races. Soon, who was also human, died of old age, but he was the oldest member of the Scribble team. Serini may well still be alive, and if Kraagor really went through to the other world instead of dying, he almost certainly is as well.
But Dorukan was a wizard, and probably had access to different life extending spells. The illusionist spawned an entire multi-generational clan in the same time period.
There is no means to extend one’s lifespan while remaining one’s original species short of epic, and even for an epic character, it’s really inefficient. If you want to live forever, you need to become a lich, or a brain in a jar, or something.
He could have just “gathered the clan”.
As I wrote above, the timeline in the series is pretty well-established. The rifts were discovered in 1117 and sealed up in 1119 (which is when Kraagor died or disappeared). The current year in the strip is 1184.
I checked the family tree to look into this point. It showed that Girard’s clan were his children and grandchildren.
Not necessarily. Remember, we’ve seen one edition transition; it’s not inconceivable that Dorukan existed under earlier editions still. First Edition AD&D had Potions of Longevity, each of which made you 1d12 years younger, with the crock of a 1% cumulative chance that each one you drank would undo the effects of all of them. Even with average rolls, you could probably add a couple hundred years to your lifespan before the crock caught up with you. WotC has an archived table showing the price at 1000gp each; in a setting with potions shops (which the Stickverse has), all it would take is money.
There have been other dodges, depending on what the DM lets you get away with. It was, I think, a fairly common house rule to allow mages to research improvements to the Permanency spell that would add new spells to the list it worked on. I know of at least one DM who allowed a player to add Shape Change to the list. Permanent Shape Change can be used to effectively give the wizard eternal youth (at least until the Permanency gets dispelled)–all he has to do is shapechange into a younger person of his species as often as necessary. Of course, the spell is extremely useful for other things, too.
And the account of the Dark One, according to Redcloak. The fact that both the Dark One and Shojo talk about the Snarl indicates that if it’s made up, the older gods are probably behind the story, not any mortal. And that they apparently killed people to make the story look real, given that the Snarl killing people & destroying their souls is part of both accounts.
He must have done something, he wasn’t young when the Gates were put up, but he was still human and still had a libido over 60 years later.
I think 3 generations count as multi-generational. And they were apparently very fertile from the number of them that were lying around dead. 
2nd Edition had the same drawback for Potions of Longevity, but none for Elixirs of Youth, which always struck me as kinda weird.
(It also allowed priests of Shou Hsing to simply stop aging at tenth level – which, actually, wasn’t that big a deal; priestesses of Idun could stop physically aging right at first level, and priests of yet another deity got to change their ages at will upon reaching 15th level. But the groovy part about Shou Hsing’s priests is that they could amiably halt the aging of others.)
There is also no means to kill hundreds of beings across thousands of miles because of their ancestry.
I would tentatively say that Familicide is safely within the Epic spell research rules. Which really says more about how ridiculous the Epic rules usually were than it does about how powerful the Familicide spell is.
More simply, there’s an official 1e / 2e 9th level spell called Life Force Exchange. Which does exactly what it says on the tin: exchanges the life forces of two people. Usually the aged wizard and a hapless youth.
Note that I said “short of epic”.
Which brings up a point. What does or should epic mean? On its face, epic should be extremely rare, as epic level casters can literally break the world. As in there shouldn’t be more than one at time and each should be remembered by the whole world until the next comes along generations later.
On average, probably, though they tend to come in clumps, due to most adventurers being in parties.
The spellcraft DCs make that a little difficult. Unless you have a whole college of magicians using Aid Another at +2 a pop.
You just need some imagination. I could use that epic teleport spell to remove at least the foundations of a city, or possibly, at the extreme, the core of the planet, with a few tries. And Familicide could depopulate nations of more randy beings than dragons. Even if you could only cast it once a day, how long would it take to depopulate Earth?
Until you accidentally cast it on that long lost cousin you didn’t know you had and then you don’t cast anything ever again.
Well maybe not Tarquin yet, but ScarfFace and the MindHead chick are totally with my “call it off because ROI sucks” theory.