DONTFORGET: The Unofficial Story Mode Novelisation Prologue

(+ a screenshot leak from time travel)

Apparently, Frisk has seen in-game height, which they share with Noel, but has no confirmed age, though they share it with Suzy.

On other hand, Harlow has confirmed age, but it's unknown what was their height in UT.

What's your opinion on age range?

Deltarune x Homestuck Classpect Theory

(November edit, even though it's a jokey detail)

As a True Gemini (born June 4th), Ness Sans the #Skeleton has dual Dream Selves which represent his past life in DELTARUNE as a rich businessman who enjoyed his life, and his current depressive life in UNDERTALE, looking in the past and trying to go back.

DELTARUNE Theory: Not only Dragon Blazers/Skies Forever Blue reflect UT/DR, but Mew Mew does it as well!

Read the article.