NEW YORK – A lost hoodie here, a forgotten backpack there. In Justin Bieber’s case, an open fly on the Grammy red carpet!

What’s with teens and brain fog?

Parenting teens and tweens can sometimes with parents asking: Why is my kid still skipping through life on another planet? When’s he going to land back on Earth?

The short answer might be later rather than sooner, at least when it comes to their brains.

Most scientists once believed the human brain reached full development by age 12. But research based on improved brain scan technology indicates that coordination of certain functions continues to come together through the early 20s, said Lawrence Steinberg, a psychology professor at Temple University.

“It’s not that a teen is forgetting,” he said. “It’s more like they’re much more drawn to the immediate reward of a situation than adults are and they’re much less likely to think ahead and think about the future. The future can be just an hour later.”

Steinberg regularly hears from parents as the author of the recently revised “You and Your Adolescent: The Essential Guide for Ages 10-25.”

“These days I get almost as many questions from parents about dealing with 20-somethings as I do about teenagers,” he said.

Brain fog is a life force at the Waterville, Maine, house of Marc Pitman. He has a simple, standing request for his 11-year-old son: Shut your bedroom door. “He’ll go to close the door, forget on the way and come back to the living room having left his door open.”

Pitman relies on “auctioneer-like repetition” of common-sense instructions.