None. There aren't many famous twelve year olds, let alone one born on that date. Here are famous people born on August 20:
Benjamin Harrison born on August 20, 1833
Paul Tillich born on August 20, 1886
Mikio Naruse born on August 20, 1905
Andrei Konchalovsky born on August 20, 1937
Jacqueline Andere born on August 20, 1938
Isaac Hayes born on August 20, 1942
Ray Wise born on August 20, 1947
John Noble born on August 20, 1948
Robert Plant born on August 20, 1948
Al Roker born on August 20, 1954
Don Stark born on August 20, 1954
Joan Allen born on August 20, 1956
Geoffrey Blake born on August 20, 1962
James Marsters born on August 20, 1962
Robert Warzycha born on August 20, 1963
Giuseppe Giannini born on August 20, 1964
Terri Poch born on August 20, 1967
Celso Ayala born on August 20, 1970
Els Callens born on August 20, 1970
David Walliams born on August 20, 1971
Fred Durst born on August 20, 1971
Oronde Gadsden born on August 20, 1971
Steve Stone born on August 20, 1971
Chaney Kley born on August 20, 1972
Todd Helton born on August 20, 1973
Amy Adams born on August 20, 1975
Shaun Newton born on August 20, 1975
Kristen Miller born on August 20, 1977
Ben Barnes born on August 20, 1981
Bernard Mendy born on August 20, 1981
Meghan Ory born on August 20, 1982
Claudine Schaul born on August 20, 1983
Ayesha Takia born on August 20, 1986
Robert Clark born on August 20, 1986
Catalina Ponor born on August 20, 1987
Liana Liberato born on August 20, 1995