The Determination injections reversed the state of “fallen down” for the monsters in the Lab. 

Heck even the remains of monsters can be brought back, as seen with Flowey, with Determination and a physical form to anchor to.

And I think I know why.

My body…It feels like it’s splitting apart.
Like any instant…I’ll scatter into a million pieces.
Deep, deep in my soul.
There is a burning feeling I can’t describe.
A burning feeling that WON’T let me die.

Monster SOUL’s have Determination in them.
But unlike humans who have bodies made of physical matter, and whose Determination makes their Souls persist after death…

Monsters use their Soul's Determination to maintain their bodily form.

That’s why even normal monsters like Gerson can live for a millennia without ever “falling down” and why monster Souls don’t last long after the Body is destroyed.
Age doesn’t cause “falling down” because the age pool of monsters is at such a wide range going from Snowy’s mom to Shyrin’s sister.
When monsters loose Determination it compromises their physical wellbeing.


Monster funerals, technically speaking, are cool as heck.
When monsters get old and kick the bucket, they turn into dust.
At funerals, we take that dust and spread it on that person’s favorite thing.
Then their essence will live on in that thing…

(Snowdin Library: school report)

It’s funny how age seems to be a vague indicator for those who have “fallen down"  when Gerson is practically one of the oldest surviving monsters who isn’t a boss monster and he’s still kicking while Shyrin’s Sister and Snowy’s mother are one of the "fallen down” while other monster’s around their age range are still fine.

Perhaps it is not only age, but the amount of physical strain a monster accumulates over time that causes their bodies to “fall.” Monster’s do have a fragile physicality that is also affected by their mental state. Being so in tuned with their Souls, I wonder if monsters can get physically harmed by their worries and heartaches?