something or someone that is not in its correct historical or chronological time, especially a thing or person that belongs to an earlier time: The sword is an anachronism in modern warfare.
Recently, I read a story wherein the heroine (age 28) referred to a television commercial that aired in the 1970s. The product itself hasn't been in existence (except through dealers in rare items) since 1976--nearly ten years prior to the character's birth. Needless to say, this jolted me out of the story.
If you're going to write characters who are much younger (or older!) than you, be sure to research carefully. Spend time with people who are the approximate age of your characters. Use search engines wisely! Be sure the slang you use, the pop culture you reference, and the knowledge you give your characters is age-appropriate.
For example, anyone born between the years 1966-1976 are considered Generation X. Generation X children were more likely to have lived through their parents' divorce than any other previous generation. They were the first "latchkey" kids. As they aged, they've become focused on higher education, and financial and family security. They tend to use digital technology to simplify their lives through online banking and shopping. Gen X watched the OJ slow-speed car chase and the subsequent trial, which was probably their first foray into Court TV. They came of age during the AIDS crisis, the stock market collapse, and the 90s dotcom burst. This was the MTV generation (when MTV still aired music videos). Their movies reflected coming of age angst: The Breakfast Club, Reality Bites, Clerks, Fast Times at Ridgemont High. On television, they tuned in to Beverly Hills 90210, Buffy the Vampire Slayer, and The X Files.
After 1976 to about 1994 are Generation Y, or Millenniums (also known as Echo Boomers since they are the largest segment of new population since the Baby Boomers generation). Generation Y kids had soccer moms and trophies for every player. This generation is more technologically savvy, thanks to the popularity of the PC and the Internet. For the most part, these are the bloggers and social media mavens. They're culturally diverse and more liberal in their thinking. Gen Y kids played with Transformers and Pokemon. They fell in love with Elmo. Pop music renewed the popularity of boy bands with 'N Sync and The Backstreet Boys, but added the hiphop of Eminem and Tupak. Their television shows were edgier: South Park, Family Guy, Two and a Half Men, as well as the glut of reality shows that burst from the networks. They grew up with Home Alone and The Lion King--on videos they could watch whenever they wanted--before discovering the films of Judd Apatow and The Lord of the Rings trilogy.
What a difference a few years makes!
To fully connect with your reader, be sure to research your characters, their pasts, and their experiences before creating their fictional journeys.