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The recent discovery of bright quasars around redshift z=6 suggests thatblack holes (BHs) with masses in excess of 10^9 Msun have already assembled ata very early stage in the evolution of the universe. An alternativeinterpretation is that these quasars are powered by less massive BHs, but theirfluxes are strongly magnified through gravitational lensing by interveninggalaxies. Here we analyze the flux distribution of the Ly alpha emission of thequasar with the highest known redshift, SDSS 1030+0524, at z=6.28. We show thatthis object could not have been magnified by lensing by more than a factor offive. The constraint arises from the large observed size, 30 (comoving) Mpc, ofthe ionized region around this quasar, and relies crucially only on theassumption that the quasar is embedded in a largely neutral IGM. Based on theline/continuum ratio of SDSS 1030+0524, we argue further that this quasar alsocannot be beamed by a significant factor. We conclude that the minimum mass forits resident BH is 4 x 10^8 Msun (for magnification by a factor of five); ifthe mass is this low, then the quasars had to switch on prior to redshift z=9.  From the size of the ionized region, we are also able to place an absolutelower bound on the age of this quasar at t > 2 x 10^7 years.Comment: Submitted to ApJL, 2 figures include

A Constraint on the Gravitational Lensing Magnification and Age of the Redshiftz = 6.28 Quasar SDSS 1030+0524