The recent discovery of bright quasars at redshift z \sim 6 suggests that black holes ( BHs ) with masses in excess of \sim 10 ^ { 9 } ~ { } { M _ { \odot } } have already assembled at a very early stage in the evolution of the universe .
An alternative interpretation is that these quasars are powered by less massive BHs , but their fluxes are strongly magnified through gravitational lensing by intervening galaxies .
Here we analyze the flux distribution of the Ly \alpha emission of the quasar with the highest known redshift , SDSS 1030+0524 , at z = 6.28 .
We show that this object could not have been magnified by lensing by more than a factor of \sim 5 .
The constraint arises from the large observed size , \sim 30 ( comoving ) Mpc , of the ionized region around this quasar , and relies crucially only on the assumption that the quasar is embedded in a largely neutral IGM .
Based on the line/continuum ratio of SDSS 1030+0524 , we argue further that this quasar also can not be beamed by a significant factor .
We conclude that the minimum mass for its resident BH is 4 \times 10 ^ { 8 } ~ { } { M _ { \odot } } ( for magnification by a factor of 5 ) ; if the mass is this low , then the quasars had to switch on prior to redshift z _ { f } \gtrsim 9 .
From the size of the ionized region , we are also able to place an absolute lower bound on the age of this quasar at t > 2 \times 10 ^ { 7 } ~ { } yrs .