Ok, we covered twice your age – how about half?
I’m 42 – born in 1968, so 21-year-olds are half that (damn, I find that scary). From the Wikipedia list of those born in 1989, we have:
Jake Lloyd
Chris Brown
Freddy Adu
Joe Jonas
Michelle Wie
Taylor Swift
Jordin Sparks
Damn kids…
Morgan Lily (you’ll of course recognize her as the young daughter in the somewhat recent film 2012)
Tatum McCann (you’ll of course recognize her as the young daughter in the somewhat recent film Click)
I feel old.
I’ve been thinking of this since the other thread. Lots of famous people are half my age , but the most important person to me is my daughter. 
1985:
Lily Allen
Reggie Bush
Haylie Duff
Zac Hanson
Keira Knightley
Frankie Muniz
Raven-Symone
Jack Osborne
Athina Roussel (a competitive show jumper who is the only surviving descendant of the Greek shipping magnate Aristotle Onassis. She has been reported to be the richest person of her age in the world, with a net worth of between $600 million and $2 billon.)
Lee Malvo
Michael Phelps
Cristiano Ronaldo
Bindi Irwin was the only child I recognised, the scary part is that a soon to be teenager was born in 1998! Does not compute
1978:
James Franco
Rachel McAdams
Zoe Saldana
Ashton Kutcher
Katherine Heigl
Ian Somerhalder
Michelle Rodriguez
January Jones
Justin Long
Katie Holmes
Kobe Bryant
I never heard of most of them.
1994:
Dakota Fanning
Justin Bieber
I’m 41, born in 1969. Half my age is 20.5, so let’s put that at 1990.
A lot of cute girls. Not so many guys that I recognise, though.
Princess Eugenie
Keisha Castle-Hughes
Kristen Stewart
Emma Watson
Kay Panabaker
Daveigh Chase
Rachel Hurd-Wood
Kirsten Prout
JoJo
Thomas Sangster
Jonathan Lipnicki
Why, thank you, you saved me having to search 
I didn’t remember we’re the same age!
Add a bunch of current rising stars of Primera División.
nothing to see here
Now get off his lawn! 
1991:
Jamie Lynn Spears
Madylin Sweetin
The only two I’ve heard of on Wikipedia’s list.
ErinPuff
also:
Mischa Barton
Amanda Bynes
Charlotte Church
Megan Fox
Lady GaGa
Shia LaBeouf
Lindsay Lohan
Mary-Kate and Ashley Olsen
1982:
Kate Middleton
Jodie Sweetin
Jessica Biel
Seth Rogen
Kelly Clarkson
Apolo Anton Ohno
Tara Lipinski/Leelee Sobieski (same date, June 10)
Prince William
Anna Paquin
Brad Renfro
Anne Hathaway
Elisha Cuthbert
Nikki Blonsky
Michael Cera
Rupert Grint
Haley Joel Osment
Rihanna
This shocked the hell out of me.
Aren’t those little children?
Half my age would be born in 1994. Wikipedia doesn’t have a very extensive list, but I’m actually finding people I’ve actually HEARD of, unlike when I looked through the first few pages of those born in 1942 and 1943…4 of them, all singers. 3 Japanese and part of the same group.
Justin Bieber
Plus half of C-ute: Saki Nakajima, Airi Suzuki, and Chisato Okai.
From the other thread I learned that Dopers RandMcnally, Maiira, and The Man With The Golden Gun are half my age. Durn whippersnappers!
Also from 1987:
Ellen Page
Ke$ha
Joss Stone
Maria Sharapova
Tim Tebow
Blake Lively
Hilary Duff
Zac Efron
I’ll be 44 this year, born in 1967, so it’s the young whippersnappers born in 1989 who are half my age.
This includes all of Hal’s list and my own charming son. Uppity kids!
I’m 25, but it’s 2011, so call that 26 years.
1998:
Elle Fanning
Jaden Smith
I’m turning 34 in a three months, so that makes half my age 17:
People I’m familar with born in 1994:
Dakota Fanning
Jodelle Ferland
Cayden Boyd
Alexander Gould (Weeds)
Liliana Mumy (Santa Clause 2 & 3)
Makenzie Vega
Dominic Janes (Alex Taggart 2.0 on ER)