Writers want to cram everything right up front.
"I know all their history, why would I want to withhold it from the reader?"
"I wrote it that way."
"It's the good part."
Writers spend lots of time imagining and writing every little detail about a character's past, be it for a child or an adult. So, of course, writers would want to tell everything right away. Perhaps, in the process, even show off a bit how clever they are. Until, one understands how curiosity works.
Not telling everything makes the reader curious. Curiosity draws the reader deeper into the story world. The reader wants to fill in the "who," "what," "how" (the "where" and "when" have already been clearly established right up front to ground the reader). They keep reading. This is good.
Tell the reader only what they need to know to inform that particular scene. This is especially true in the Beginning (1/4 mark). During the first quarter of the project, the character can have a memory. But, for a full-blown flashback, where you take the reader back in time in scene, wait until the Middle.
(PLOT TIP: If you're absolutely sure you absolutely have to include the flashback, try using one when you're bogged down in the middle of the middle.)
Click on green highlighted plot concepts for further explanations via video. Each time a concept is referenced you are directed to new information about the Universal Story and writing a novel, memoir or screenplay.
Visit Plot Series: How Do I Plot a Novel, Memoir, Screenplay? on YouTube. A directory of all the steps to the series is to the right of this post. Enjoy!