Is Dylan immature in his response on day 1 - Yes - he has some growing up to do and he is the baby of the family. (Isn't this a reasonable author's way to show starting behavior and then, personal growth - by giving someone a clearly immature place to start from?). Does he start to show more maturity as the story evolves - Yes - IF you make those choices.
1. Do we have the choice to have Dylan tell his friends to cut that crap out when they are talking smack about Sophia - Yes (standing up for his family - I'd call that a more mature response).
2. 1st Dylan/Ellie bonus story (only available if Sophia takes the Aiden path - so no Aiden path, no ability to show of Dylan's personal growth), he offers to fix Ellie's laptop that has a malware problem and all he wants is a hug and kiss from Ellie (he turned down her offer of $ - so he valued the hug and kiss more - another mature response).
In my Sophia story, she decided not to try to hook up Dylan with Emma - so Dylan continues his story with Sophia - and there is no dialogue calling Emma anything. In the playthrough others are quoting, they have chosen options that have Dylan respond in an immature way. So - is Dylan growing in maturity or is he remaining an immature kid? It depends on your choices in the same way that if Sophia chooses the good wife path and shuts down anything to do with any man other than Liam, she continues to maintain her focus on her marriage with Liam - all choice driven.
So... saying Dylan is immature because someone makes immature choices for him is kind of self-fulfilling. Those making choices like that are getting the Dylan they envision. I am getting the Dylan I envision because I made different choices. Everyone gets to have the Dylan they CHOOSE. I don't understand the repetitive arguing just because some of us make different choices. If I say that today is Thursday, October 22nd, 2020, and you say but tomorrow will be Friday, October 23rd, 2020 - we are Both right! I'm choosing to focus on today and you are choosing to focus on tomorrow, both choices are equally valid. In AWAM, we get to make the choices to tell the story we envision - that is part of the beauty of AWAM. Disengaging soap box in 3...2...1...