The sad fact is that most medical schools assume that all medical students are 22-years-old and fully supported by their parents. Here are a couple of schools' stated policies on institutional financial aid:
From Dartmouth Med's financial aid web site:
"DMS, like most U.S. medical schools, requires parental income statements from all applicants. This requirement cannot be waived even though a student is independent of his or her family."
http://www.dartmouth.edu/~dms/dms_admiss/admiss_cost.html
Rochester Med:
Institutional Policies Regarding Parental Support
"It is the policy of the University of Rochester School of Medicine and Dentistry to expect parents, to the extent of demonstrated ability, to contribute to the medical education of their children. For the purpose of establishing eligibility for University assistance, no medical student is considered to be financially independent of his/her family regardless of age, marital status, other graduate degrees, or the fact that he/she may have been self-supporting for a number of years. Parental income information must be submitted via Need Access as part of the application procedure for any student who seeks School or School-administered funds."
http://www.urmc.rochester.edu/smd/finaid/MDhandbook/Handbook2000.html#18
A handful of medical schools will allow you to declare yourself fully independent if you provide enough proof. Stanford is one school that recently made this change "Students age 30 as of the first day of the academic year; parental data will no longer be required in determining SU grant eligibility."
http://www.med.stanford.edu/osa/financialaid/
I think Duke is another one of the few schools that considers requests for independence on a case-by-case basis.
Other than that, you're stuck with taking out extra loans if you can't provide your parents' data. I had the same problem- I got accepted to a highly ranked private school that offered decent financial aid, but invariably required parental data to qualify for grants. I'm 30 year old, married and have been fully self-supporting for over a decade. It was extremely awkward asking my parents for their financial information and I eventually gave up entirely. Rather than take out extra loans to cover the cost of the private school, I chose to attend an inexpensive, unranked state school.