Before you begin the story, here's some things to know.
1) Frisk is a girl.
2) Chara is a girl.
3) Mettaton is a boy.
4) Alphys and Undyne are not in a romantic relationship. They are merely friends. (The two didn't work out. However they maintain a close friendship.)
5) Frisk (and Chara) are 17.
6) Toriel is 55.
7) Asgore is 57.
8) Sans is 21.
9) Papyrus is 19.
10) Alphys is 22.
11) Undyne is 24.
12) The story takes place 3 years after the True Pacifist run. However, a Genocide run was done beforehand.
13) Frisk mainly communicates through sign language, though she can talk.
While most of the monsters can be presumed to be very old, the ages previously stated is what I believe their physical age would be in human terms.
Gentle reminder to all that this is a fanfiction. Not everything that happens in this world is in accordance to the original Undertale. This is my interpretation, my completely not cannon story, my extremely fictional piece of literature. If you don't like it, don't read it. Please and thank you! I hope you enjoy this rendition of Undertale. :)