Outliers. Most of the books written by younger people who have no experience in life will sound immature to anyone above their age.
How many of those great books written by over 40yo authors had never written books before that age allowing them to develop their writing skills?
Goodness, if only someone compiled an article's worth of authors who only first got published after 35. Maybe around 35 of them would be enough to answer your question.
Writing books is how you get good at writing books, and young people read books too, so it's no loss if an older reader feels it's immature. Perhaps in their wisdom, the older reader could find some empathy for the younger writers current lens that they are writing through.
Good writers are outliers, most books will sound immature. When the author's gatekeeping would get rid of much of Shakespeare's output, maybe it's too much?