When does the Secondary Tax Rebate Kick in: During the birth month or the tax year one turns 65?

I am in my 65th year... ie. I am currently 64 but will be turning 65 in September 2016. This means that when my final 2016/17 tax return is submitted to SARS at tax year end I will technically be entitled to the secondary rebate for the age group 65 - 75. Should my employer calculate my monthly PAYE Deductions at my current age (64), or the age I will be at the time of this final tax submission (65)?

Yes, your employer should take this into account when calculating your rebate to come as the law applies to the last day of assessment.