Is age a factor when applying for federally-funded pre-doctoral fellowships (such as NIH's F-31 grant)?
The NIH offers a bunch of comparison data of NRSA recipients.  I think you should  pay considerable attention to figure 2.9

Only about 55% of fellows receive Ph.D.'s by 30.  

There is more data to be mined from the entire report -- and it shows that about 10% of NRSA recipients received their degrees at 36 or older.  Given what I think the applicant pool must be, I suggest that age is likely not a bias.

Also, the mission statement for Kirchstein states that:


  The overall goal of the NIH Ruth L. Kirschstein National Research
  Service Award (NRSA) program is to help ensure that a diverse pool of
  highly trained scientists is available in appropriate scientific
  disciplines to address the Nation's biomedical, behavioral, and
  clinical research needs. NRSA fellowships support the training of
  pre-and postdoctoral scientists, dual-degree investigators, and senior
  researchers.  More information about NRSA programs may be found at the
  Ruth L. Kirschstein National Research Service Award (NRSA) website.
  
  The purpose of the Ruth L. Kirschstein National Research Service Award
  (NRSA) Individual Predoctoral Fellowship (Parent F31) is to enable
  promising predoctoral students to obtain individualized, mentored
  research training from outstanding faculty sponsors while conducting
  dissertation research. Applicants for this F31 program are expected to
  propose a dissertation research project and training plan in
  scientific health-related fields relevant to the mission of the
  participating Institutes and Centers. This training plan should
  reflect the applicant’s dissertation research project, and facilitate
  and clearly enhance the individual’s potential to develop into a
  productive, independent research scientist.  The training plan should
  document the need for, and the anticipated value of, the proposed
  mentored research and training in relationship to the individual’s
  research career goals.  The training plan should also facilitate the
  fellow’s transition to the next stage of his/her research career.


As with any grant, you should ask yourself whether your situation matches the institutional aims.  I would think the more "atypical" your application package is, the more you need to consider this. 

Age is a balancing act.  The older you are, the more mature you are, and the better you should be able to describe your aspirations and career goals.  If you look like you're putting off hard decisions, your application will suffer.  This is true regardless of age, but probably especially true for older applicants.