David,
First of all, age is not specified in your model:
-model "1+Time+TrialDisplacement+TrialDisplacementSquared" \
The question is, do you want
-model "1+Time+Age+TrialDisplacement+TrialDisplacementSquared" \
or
-model "1+Time*Age+TrialDisplacement+TrialDisplacementSquared" \
> I would like to estimate the HRF time course at a particular age, in this case age 18, for an idealized zero-motion case.
With
-model "1+Time+Age+TrialDisplacement+TrialDisplacementSquared" \
you don't really need to worry about the center values of those quantitative variables since no interaction effects are assumed, so you can simply go with
-gltCode 1 'Time : 1*T00'
Gang
