No, there is no income limit when you are receiving Social Security benefits and you are at Full Retirement Age.
Yes, your earnings are subject to Social Security and Medicare taxes regardless of your age.
Your Social Security benefits can be taxable based on your other income and your filing status.
Up to 85% of Social Security Retirement/Disability/Survivors benefits 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