Is this credit applied just to pension and investment income?
Before age 70, I thought the 50% penalty on Social Security benefits kicked in around $15,000 on income from wages and salaries and self-employment.
The credit applies to those over the age of 65, or those who are totally and permanently disabled. The credit is limited by taxable and nontaxable income. Whether or not 50% or 85% of social security benefits are subject to taxation was discussed in a previous post (the one referred to in the first sentence of this post) which you may find interesting >>
Taxing Social Security Taxes.