Hit the library to check out as many books as you can from the shelves that target your readers' age-range. Then read, read, read, and never stop reading! Become familiar with the format, the sentence structures, the way the opening chapters read, and how quickly the plots develop.
In addition, there are writing "norms" for each age range: Picture Books, Chapter Books, Middle Grade, Young Adult, and those norms are constantly changing and evolving. Read to keep up with these changes, to know they're out there - and decide if you want to revise according to what you're reading. Reading won't take away from your writing and creativity - it will enrich it, give you ideas, begin to shape the way you write, and inspire! But always, stay focused on how you can use what you read to write and revise your book.
In the interim, I read - voraciously, non-stop. I taught books. I can now read the opening pages of some manuscripts and identify writing styles from 30, 40 years ago that aren't published any longer. Reading enables you to identify the range of what's currently being published, and where your writing falls in that range.
Then, if you deliberately choose to write in an older style, know that you're doing that, and your chances of publication are diminished - although nothing is impossible!