The Centers for Disease Control and Prevention produce data on health and healthcare in the United States.  Health, United States includes a variety of tables with breakdowns by poverty status.

In 2012-2013 their data for children under 18 with “no usual source of healthcare” show

• 4% of all children had no usual source of healthcare
• for poor children the rate was 6%

Children under 6 were less likely than those 6 to 17 to have no usual source of healthcare (3% vs. 5%).  Uninsured children were ten times more likely than insured children to have no usual source of healthcare (28% vs. 2%).  For uninsured children under 6 the rate was 20% and for those age 6 to 17 the rate was 29%.

For adults ages 18-64 data on “no usual source of healthcare” show

• 20% of all adults ages 18 to 64 had no usual source of healthcare
• the rate for poor adults in that age group was 33%
• adults ages 18 to 44 were less likely than those ages 45 to 64 to have no usual source of healthcare (25% vs. 12%)
• The rate was highest for uninsured adults at 54%
• The rate was lowest for insured adults at 11%

In 2013, data on preventive care show that

Sixty-seven percent of all women ages 40 and over had a mammogram.  The rates vary by income level:

• 50% of women at or below the poverty level
• 57% of women between 100-200% of the poverty level 
• 77% of women at 400% or more of the poverty level

Fifty-five percent of adults 50 to 75 had colorectal tests or procedures.  The rates vary by income level:

• 41% of adults at or below the poverty level
• 45% of those between 100-200% of the poverty level
• 63% of those at 400% or more of the poverty level

Source:
National Center for Health Statistics. Health, United States, 2014. Hyattsville, MD., 2013. Tables 712-73, 83, 85 (PDF) Accessed 11/12/2015