What about this list found there :
http://www.tennisnews.com/exclusive.php?pID=18975
TEENAGERS (18) IN ATP TOP 10 RANKINGS
Since the inception of the ATP Rankings on Aug. 23, 1973, there have been 18 teenagers to rank in the Top 10. Half of them went on to rank No. 1. Djokovic is the latest addition to this impressive list.
Age Broke into Top 10 Career High
Krickstein, Aaron (USA) 17 years, 11 days 6 (February 26, 1990)
Chang, Michael (USA) 17 years, 3 months 2 (September 9, 1996)
Becker, Boris (GER) 17 years, 7 months 1 (January 28, 1991)
Wilander, Mats (SWE) 17 years, 10 months 1 (September 12, 1988)
Borg, Bjorn (SWE) 17 years, 11 months 1 (August 23, 1977)
Agassi, Andre (USA) 18 years, 1 month 1 (April 10, 1995)
Medvedev, Andrei (UKR) 18 years, 9 months 4 (May 16, 1994)
Nadal, Rafael (ESP) 18 years, 10 months 2 (July 25, 2005)
Arias, Jimmy (USA) 18 years, 11 months 5 (June 25, 1984)
Ivanisevic, Goran (CRO) 19 years, 11 days 2 (July 4, 1994)
Sampras, Pete (USA) 19 years, 29 days 1 (April 12, 1993)
Edberg, Stefan (SWE) 19 years, 1 month 1 (August 13, 1990)
Hewitt, Lleyton (AUS) 19 years, 2 months 1 (November 19, 2001)
Cash, Pat (AUS) 19 years, 3 months 4 (May 9, 1988)
Carlsson, Kent (SWE) 19 years, 5 months 6 (September 19, 1988
McEnroe, John (USA) 19 years, 7 months 1 (March 3, 1980)
Djokovic, Novak (SRB) 19 years, 9 months 7 (April 2, 2007)
Roddick, Andy (USA) 19 years, 11 months 1 (November 3, 2003)
... and we should add
Murray, Andy (UK) 19 years,11 months 1 (April 16, 2007)
So then they are 19 players under 20 y.o to have reached the top ten.