Maybe a dumb question, but why do you think Ben Solo is 29 in the ST? Do you think it's because they decided his age around his actor's? Because I get that the coming-of-age themes in this story are more metaphorical than literal, but I also wonder if fans would be more accepting of Kylo's disposition and his emotionally stunted behavior if he were around Rey or Finn's age? I'm going off Rian's comments saying Kylo represents the anger of adolescence and choosing who he will become as a man.
I think they tried to stay as true to the actors’ real ages as possible, and outlined the timeline accordingly. While Adam can and does look younger in TFA than Ben’s canon age, there’s no way Carrie, Harrison and Mark could pass as younger than 50-55, so the sequel trilogy *had* to be set circa 30 years after RotJ. Now, they could have had Ben be born later, around the time of Finn’s birth if not Rey’s, but then the symbolic significance of his being conceived shortly after the battle of Endor and born as the last remnants of the Empire dissolved would have been lost.
Plus, there’s no time gap between TFA and TLJ, so Adam is currently already 3-4 years older than canon Ben, and the age difference would be MUCH harder to conceal if they were trying to pass off Ben as 23-24.
(this also tells you how much they wanted Adam for the part, imho)
In any case the characters’ canon ages are a piece of trivia that the general audience isn’t supposed to *know* or care about. All it matters is that Kylo looks the part of an age peer to Rey and Finn, and thematically and narratively he is their age peer.
And modern audiences are more than used to coming of age stories involving older characters anyway. We live in an era where adolescence bleeds into adulthood for much longer than it used to be. Due to the economical crisis and other factors like higher education becoming the norm and excessive social competitiveness, young adults in western countries are often insecure, confused and struggling to find their financial independence and place in the world. This is why Rian & co. say things like “we can all relate to Kylo”. [side note, Tumblr culture is weirdly fixated on literal, schematic definitions of “adulthood” and “underage” that don’t leave room for transitional periods, non-neurotypical development and grey areas (like, how is it supposed to work? you’re a “literal child” until 18 then you magically level up to “actual adult”?), hence the “Kylo Ren is LITERALLY AN ADULT what is this adolescence nonsense” wank, which is incredibly ironic imo considering that the majority of tumblr discoursers is in the 18-24 age bracket yet they still act and stomp their feet like spoiled children, and sometimes demand to be treated as such.]