Rambles about utdr humans n stuff enter at your own risk lol
I know its almost complete consensus that the fallen humans were all children, (could only find like 2 citations for this) but with the new context of Deltarune now I can't. Like in all thought, why would the yellow child reasonably have a revolver? There's no disambiguation like the toy knife that the empty gun is anything but a real firearm. Deltarune gave a pretty decent, maybe more accurate "age" of the fallen humans:
Yellow and Blue being the oldest, being at least young adults, able to own firearms and being physically strong enough to provably kill some monsters in a more physical altercation. (Gun wouldn't lead to dust on you since distance, but the tutu being dusty means a monster was killed in close proximity.) The former getting the farthest to presumably Hotland which would be possible if y'know. You had a gun, and a very real mental reason (in assumption they fell together) to kill monsters at that point, making it easier for them to kill. Running out of bullets was probably the only reason he never made it to New Home.
Aqua and Orange being young children, one exploring the mountain out of sheer curiosity the other to prove they're brave enough. Aqua never getting very far since she was very reckless and outwardly violent in some way to other beings. Orange only getting barely out of the ruins but not enough survival instincts to make it through Snowdin as soon as they're out of the protection of Toriel.
Seth and Green being somewhere in between, maybe teenagers, Seth getting to almost the exact halfway through strategy with some critical thinking ability, but not past waterfall depending on how long ago they fell, falling to Undyne or Gerson in his prime (likely gerson considering hes where you get the items). Green getting second farthest through just sheer kindness, though seems something broke them down in Hotland to the point BOTH their items are just... scattered on the ground.
Into HC territory heres what I see how/what/when lead them to the underground (assuming the ruins entrance is the only entrance).
Blue: Fell with Yellow together, Blue being in full ballet dresswear assumes he was specifically on the mountain relating to it. Something like taking photos or a performance nearby and walked away to get some air. Maybe needed a ranger (Yellow) to accompany him.
Yellow: Fell with Blue together, being armed during his time in the underground makes me think some sort of park ranger or security, not an actual cowboy unless he fell in like the 17th-18th century. (though this could still line up with blue as ballet was quite established in the late 1700s) Sadly we don't have a concrete timeline for each human, but likely fell within semi-recent history for each.
Aqua: Likely just a child that just snuck away where she wasn't supposed to. Plastic wasn't widely used in toys until the 1900s so she fell quite recently to Frisk. Not even making it to Home, how they got her soul out of the Ruins is up in the air but one can assume Toriel was involved in finding her eventually.
Seth: Curious teen taking notes about the flora and fauna of the mountains, researching and just wanting to catalog things. Book smart, but not socially smart in a lot of ways, could strategize around monsters but never could do anything if physically confronted. Notebooks and glasses have existed for a very long time, which would line up with being as early as the 13th century with the invention of consumer glasses. With how old Gerson is, living since the underground was sealed, its very likely the cloudy glasses could mean unrefined glass or very old.
Green: Honestly the one I'm most, "I dunno" about. Aprons have been around forever, so have been pans, Green could have fallen at almost any time in human history. All we know is they were a chef and put being nice ahead of everything.
Orange: Besides Aqua likely the youngest, or at least smallest, of the humans. Bullied and made fun of for that fact and decided she'd climb the mountain to prove she was brave. It is interesting how in Deltarune they gave her boxing gloves while Undertale it was noted to be a 5 finger glove though. Though her in general is a subversion of expectations as a character, so it does stay in theme with her not completely being 1:1 to her representation items.
