First let me clarify, by youngster, I mean 18 and under, so born in 1994 or later. I thought this might be a fun thread, since not everyone keeps track of junior tennis or the ITF pro circuit regularly, so we can share our different insights.
Here are my 5 to watch (in no particular order):
1. Eugenie Bouchard (CAN)
Birthdate: February 25, 1994 (18 years old)
Reasons to watch: Current Junior Number 2, Junior Highlights include Wimbledon Singles and Doubles Champion; Improved senior ranking from 305 to 148; Senior highlights include a 50k and 25k title, a 50k runner up, QFs at WTA Citi Open, and R32 appearance at WTA Rogers Cup.
2. Madison Keys (USA)
Birthdate: February 17, 1995 (17 years old)
Reasons to watch: At 14, she became the 7th youngest player to win a WTA tour level match; Improved her senior ranking from 273 to 138; Highlights include: a 50k and 75k title, two 50k semifinals, a R32 appearance as a qualifier at the WTA Sony Ericsson Open.
3. Donna Vekic (CRO)
Birthdate: June 28, 1996 (16 years old)
Reasons to watch: Improved her senior ranking from 395 to 109. Youngest player to reach a WTA Final since Tamira Paszek in 2006. Junior highlights include Wimbledon QFs. Senior Highlights include: Runner up at WTA Tashkent Open, two 25k titles, three 25k runner ups, and a 100k semifinal.
4. Carina Witthoeft (GER)
Birthdate: February 16, 1995 (17 years old)
Reasons to watch: Improved her senior ranking from 413 to 222. No points to defend until April. Highlights include win over Tsvetana Pironkova in qualifying of the WTA Porsche Tennis Grand Prix, two 25k titles, one 25k runner up, and a 75k+H SF.
5. Ashleigh Barty (AUS)
Birthdate: April 24, 1996 (16 years old)
Reasons to watch: Improved her senior ranking from 679 to 183. Former junior World number 2 and Wimbledon Champion. Highlights include: a 50k win, three 25k wins, two 25k runner ups.
Honorable mentions: Laura Robson (18, WTA #53); (I didn't include her because she's been on everyone's radar since 2008 and everyone knows to keep an eye out for her in 2013 due to her strong finish in 2012); Annika Beck (18, WTA #77); Yulia Putintseva (17, WTA #125); Irina Khromacheva (17, WTA #189); Taylor Townsend (16, WTA #501, ITF Junior #1); Francoise Abanda (15, WTA #611, ITF Junior #8).
With the exception of Barty (who I haven't seen play since early 2012, so this may have changed) I picked girls with fairly big, powerful games who I think will see a lot of success with their style of play. Have fun with this thread... I'm interested to see who everyone has their eye on for the 2013 season!