Though deltarune is an alternate universe where monsters dont go underground and the whole war with humans never happened
It still amuses and weirds me out that if Asriel where alive in Undertale just like in Deltarune, he is considered to be the same age as burger pants, catty, and bratty who looks like they are in their early 20s (assuming that monsters age the same way as humans do). So technically if he was alive in Undertale he would meet frisk at that age so the events of ut would probably be like 10 or more years after he was killed. It also explains why his God of Hyperdeath form looks older rather than just a design choice to make him look more like a boss in the final battle. (Or maybe the reason is both)
Another thing to point out that amuses me is the age gap of Kris and Asriel VS Chara and Asriel are fairly different. Though its not explicitly stated, most assumed that Asriel and Chara were about the same age or at least near albeit because of the arts drawn in the game and how they interact with one another (ex. Asriel being more passive and seem to rely on Chara more).
Kris on the other hand is confirmed being the younger child, as Asriel is already off to University (assuming that its the first or second year since Toriel and Burgerpants seems to implies this but again not stated) while Kris is still in either highschool or near the end of elementary. Kris seems to rely much on Asriel as told by Toriel (Asriel carrying them on his back, their visits at the coffee shop after their parents implied divorce, and the ball of junk which has some sort of sentimentallity between Asriel and Kris).
So if we compare both Undertale and Deltarune, Chara would be older than Kris and Frisk would be younger than them
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