For 2007, the standard deduction for a single person over 65 presuming he is not blind or a dependent is $6650. If he is married filing separately, $6400.
Add $3400 to the standard deduction to find the filing requirement (assuming no self-employment income), which is also the "tax free". However, certain additional deductions and credits may lead to a lower tax liability.
You can view filing requirements and standard deduction amounts in this publication:
I apologize, your original post did not mention a couple married filing jointly.
If you are married filing jointly, and both spouses are over 65 and not blind, then the standard deduction is $12800. If only one spouse is 65 or over, then the standard deduction is $11750.
Filing requirement for both spouses 65 or over $19600, and only one spouse 65 or over is $18550.
Does my answer address your concerns, or do you need additional information?