He always tries to have plausible deniability (wrt Patreon's rules) when it comes to character's ages as he said here (2017)
What we know from the story as it has been told so far (in the future we can expect adjustments) is that Dylan is one year younger than Ellie and that she is going to high school
Python:
"This is Elizabeth (everybody simply calls her Ellie), Sophia's and Liam's gorgeous, sweet daughter. She's slightly more than a year older than her brother and..."
...
"She's going to be going to a high school at the other end of the city, which has one of the best football teams in the state, as well as a great cheerleader team!"
and she is going to college next year
Python:
s "So, this is my daughter, Ellie! She'll finish the high school in a few months and will be going to college! She's already got a bunch of acceptances!"
So while we have the relative age difference clearly stated there is never a clear reference to the real age in the text. Educated guesses set Ellie as a 18yo because of the reasons mentioned by
Alley_Cat and other
reasonable estimations that factor in Sophia's age and marriage date.
Last but not least, from v0.185 we know that Dylan is soon turning... 1X
Python:
s "By the way, your birthday's coming up soon."
...
dy "Mom, I'll turn 1X, not 7!"