Minimum age criteria to file income tax return is one day.
Tax payer can file income tax return from financial year he born and onward.
eg. 1)Taxpayer is born on 15 Jan 2012 then he can file his tax return from FY 2012-13 (AY 2013-14)
2)Taxpayer is born on 31 Mar 2016 then he can file his tax return from FY 2015-16 (AY 2016-17)
Answer given by Amol is correct to some extent, but once you file the return you need to sign the ITR-V acknowledgement and approve the filing. And you cannot sign the ITRV unless you become major and per the Indian Contract Act.
Hence practically you can file the return as soon as you cease to be a minor.