So I'm exhausted and about to sleep, but I decided now is the best time to talk about how I approach Monster Aging in my fics. Hopefully this is coherent. If not, ¯\_(ツ)_/¯.

ANYWAY, I view monster aging as kind of the same as Sim aging. Their childhood and adolescence is pretty close to a human's aging, because their magic is growing at an alarming rate and shaping them into the monster they're about to be.

BUT, at around age 20, their magic stabilizes. Since they're made of magic and dust, there's very little physical wear and tear, so it takes quite a while for them to visibly age. So throughout their young adulthood, they age at a rate of one developmental year to 10 actual years. At middle age (30-ish) the developmental years lengthen because they're at their most stable. I'd say about 20 years per 1 developmental year. It quickens again as they reach old age (60-ish), back to 10 years, because their magic is beginning to break down and disperse. So an average age of a monster would fall somewhere in the 900s.

Naturally, there's elements like strength of SOUL, physical composition (even with just dust and magic, different monsters are adapted to different living conditions), and pure luck. Gerson, for example, is old as balls because he has a strong SOUL and a similar physical makeup of an animal that can get to be old as balls. 

Boss monsters, obviously, are an exception.

Skeletons have a bit of a longer life span, simply because they require much, much less to survive. Since they are the bare minimum in terms of physical composition, they have much less wear and tear on their bodies from average living. However, because they are the bare minimum, they are also much more susceptible to being weakened by stressors. A skeleton with a strong constitution, though, can live for well over 1000 years. (Grandma Gaster, for instance, was 1200 when the village was attacked; had that not happened, she likely would have had another 50-100 years left.)

So in "Reset", Gaster is up in the high 900s; roughly 70-ish in developmental years. Old, but not about to keel over (just the right age for being sucked into the Void). 

Sans is around 24, though he hadn't changed much developmentally over the past few years (save for having his mouth stuck).

Papyrus is juuust starting to stable out at 18--enough for him to be considered an adult, but there's still room for improvement.

Also, for a silly little reference, Gaster, Toriel, and Asgore are all the same age (20) when they meet; since they are boss monsters, Toriel and Asgore are a bit more put-together than Gaster is. However, they stop aging when Asriel dies. That'll be explored later in the story.