18/2/2022 · If we’re to follow the same theory we enforced on Papyrus, there’s a chance Undyne is only 24 years old during the events of the game. Alphys Alphys is the Royal Scientist and the creator of both Flowey and Mettaton. She is a brilliant scientist, especially for her age. Or at least, her age that we believe.
Undyne ( /ˈʌnˌdaɪn/ UN-dyne ),  known as StrongFish91 on the UnderNet, is a fish-like monster that leads the Royal Guard. Clad in her full suit of armor, she pursues the protagonist through the entirety of Waterfall and is frequently evaded or inadvertently thwarted by Monster Kid . Contents 1 Profile 1.1 Appearance 1.1.1 Normal
That means Toriel, as well as Asgore are well over 1000 years old. Undyne: 25-35. Undyne is the captain of the royal guard which places a high responsibility on her shoulder which would only be given to somebody of around this age and must've taken at least half a decade to meet the qualifications.
Toriel (Toriel is obviously old because of being alive around all the events that happened. All we know shes most likely 40 - 60 Asgore (Same as Toriel) Alphys (Probably a little older then undyne) Chara (Probably around the same age of frisk when they died) Asriel (5 - 14 around the same age of frisk as well)
I'd say Sans is in his late twenties, since I can't imagine him to be very old. Papyrus is about 17 to 20. He has a job, he takes care of his brother, but he's also a bit of a child. Undyne I'd put as a few years older than Sans. Since she's a captain, I imagine she's had time to work on that. She's probably 35 at most. Alphys is about 28 or something.