It doesn't explicitly say what Ellie and Dylan's ages are, anywhere in the game. But that's part of the rationale for Dylan being underage.
Dylan can't be 18, for the reasons I was saying before. The one thing it does say in the Introduction, is that Dylan is a year younger than Ellie. Physiologically, he has to be at least 9 months younger, probably closer to 10. But a year is a nice round number, so easier to deal with. And Ellie is the one that is at most 18, since she is the one that is stated to be a few months away from finishing high school. She might only be 17 herself for all we know, but that makes Dylan's age possibly even more disturbing.
If Dylan was 18, and Ellie was 19, Ellie would be the same age as Nathalie. Who says she is 19 and in college when she introduces herself to Sophia. And Sophia makes no mention of Nathalie being the same age as Ellie. And Ellie isn't in college, but in high school.
And if Dylan was 18 there would also be no reason whatsoever to not mention the ages of Dylan and Ellie in the introduction, unless L&P was hiding it, because he's doing something that would be looked down upon and make people uncomfortable. So he glosses over it, and the only reason people would notice is that there are people like me that look into things like this.
With similar logic for Sam being the same age as Dylan, since Jules will be the same age as Ellie. And Zac being the same age as Dylan, since they're both on the same soccer team, which is organised by ages.
