I think we're biased by the fact that old-fashioned writing style always sounds more fancy and formal. I don't know how much kids' writing you've read recently (school writing, not social media), but I reckon most reasonably smart 10 year olds today absolutely could write like that. My daughter is 10 and (parental bias notwithstanding) she's smart but not genius-level, and she has writing that complex in her school books.