Social security benefits being taxed after 78 years old

Your age is not a factor in determining whether your Social Security benefits are taxable or not.  It is based on the other income reported on your return.

If Social Security is your only income, then it would not be taxed on your Federal return.  If you are reporting other income or filing a joint return and your spouse is reporting other income, then at least a part of the Social Security may be taxed.

Here is a link to the Social Security Administration web site that will give you some good information about what amount of your Social Security benefits may be taxable:  Benefits Planner - Income Taxes and your Social Security Benefit

First, take a look at your Adjusted Gross Income (AGI) on your return.  Add your AGI and any non-taxable interest and one-half of your Social Security.  This is your combined income.  

If your filing status is Single or Head of Household and your combined income is:

• Less than $25,000 then none of your Social Security is taxable
• Between $25,000 and $34,000 then up to 50% is taxable
• Greater than $34,000 then up to 85% is taxable

If your filing status is Married Filing Joint and your combined income is:

• Less than $32,000 the none of your Social Security is taxable
• Between $32,000 and $44,000 then up to 50% is taxable
• Greater than $44,000 then up to 85% is taxable

If your filing status is Married Filing Separate, then your Social Security is taxable.

If your benefits included a lump sum payment for prior years, then the calculation is much more complicated than stated here.  

You might be thinking of your SS not being reduced.  

There are 2 different things to know about social security. People get them mixed up all the time.

1. Your actual SS checks

If you are over full retirement age your actual ss checks won't be reduced. Otherwise they will actually reduce your payments if you make too much other income in the prior year.  See SS FAQ for working after retirement

https://www.ssa.gov/planners/retire/whileworking.html

2. Income Tax

For any age up to 85% of Social Security becomes taxable when ALL your other income plus 1/2 your social security reaches:

Married Filing Jointly: $32,000

Single or head of household: $25,000

Married Filing Separately: 0

Or are you asking about the self employment tax to pay Social Security and Medicare?  No age limit on that either.  If you have earned income you pay the FICA.  But you only pay the 6.2% for SS on $137,700 max of W2 and Schedule C Net Profit.