Kara,
One problem with your script is that the variable 'age' is quantitative: Without an explicit declaration, 3dMVM simply treats it as a categorical variable (factor). So, add the following in your script:
-qVars 'Age' \
I would like to take the opportunity to recommend Justin Rajendra's nice auxiliary tool for 3dMVM that may assist in the script constructing process:
[
afni.nimh.nih.gov]
Gang