Ooooh, I know this is old as heck, but I like to engage in fandom talk like this, and I want to step up and point out somw things I noticed:
Let's starts from the most obvious one, Papyrus. He can't be 20s at game's time, because that would mean he was alive and there to see not only the first fallen human, but all other 7 afterwards, too, seeing that if he was supposedly born in 1995, then he would have been around to have lived in the Underground during Chara's arrival, who fell in 201X, therefore, making him older than his 20s by Frisk's arrival, what with other 6 kids falling, going through misadventures, and dying, the fact that apparently there was a noticeable gap in time between Frisk's fall and the 7th kid's.
Furthermore, Sans being there before the 1st fallen human is also impossible because 1. Sans' promise to Toriel of protecting a fallen human if he meets one, evidence points to the fact that Frisk was the first fallen human he had personally met (and therefore, why he would then proceed to try to keep the promise). Not only that, Toriel makes him promise that because she had already seen all previous children's fate, so by the time Toriel and Sans met, all previous fallen humans must have died, and 2. Sans and Alphys acquaintanceship. Alphys started her determination experiments with the SOULs, so of course the kids where dead, and, although we do not know the how, we know Sans was involved somehow.
Also, the Skelebros aren't native to the Underground, they just mysteriously appeared and started to live there one day, and correct me if I'm misremembering, but wasn't Frisk the first human Papyrus had ever seen? That would mean the brothers arrived after the 7th kid's death. Sans might've seen humans before arriving in the Underground, with him being as he is, he might've know how they looked, but taking Toriel's promise and his involvement with the SOULs experiments into account, the first one he met in the Underground could only have been Frisk.
Not only that, weren't they also the first human Undyne, Alphys(who knew how they looked, but hadn't seen one in person) and a whole lot of monsters had ever seen? I remember most monsters didn't even realize Frisk was human. Therefore, most of 'em must've been born after 201X, debunking that the numbers in their undernet handles are the years they were born. Unless we take it as in 20XX, for example, Papyrus, instead of being born in 1995, was actually born in 2095.
Finally, monsters seen to age vastly differently than humans here. I've seen people assigning Toriel and Asgore as thousands of years old, but giving all the rest of the monster cast normal human years, when the fact is, it's most likely that they all a have non-human age system, for example, let's say that Papyrus really was born in 1995, but that would make him at least 30 something by game's time, or 10000 year old, or whatever number, who knows the time transpired between the fall of the children, but mentally, he is like, 16-22 maybe? We only know for sure he is younger than Sans.
Heck, even Monster Kid, he might be the monster equivalent to Frisk's age, but that might as well be 300.
People like to think that Asgore and Toriel somehow act as if they were ancient beings, while no other monsters do, but the fact is, none of the characters truly act as if they were centuries old creatures, even though they could very possibly be. Asgore and Toriel act as if they were middle-aged parents, then, if people were to apply the same logic they use for the rest of the monsters (their personality reflects their age), then they would be 40-50. Or every monster ages like humans do, or none of them do, that cherry picking is kinda weird, specially because people tend to give the king and ex-queen millennium aging system, but give their son a standard human one, when it would make more sense if Asriel was 100~, and that translated to 10~ in human years.
Anyway, do correct me if some of my points are wrong, as I said, I like fandom engagement, would love to hear others' opinons about this. If some of the stuff I wrote reads a bit weird, I'm sorry, English isn't my first language 😅