> -model 'Task*age_c*constant+Task*age2*constant' \

Both age and 'constant' are quantitative variables? If so, it's better to center each properly and then create a new variable for their interaction. Is 'age2' supposed to capture the quadratic effect of age? If so, perform centering before the squaring step.

> would it be correct that I would interpret the F test for Task*age as controlling for age2

I'm not so sure that 'controlling' is the right word to describe the situation, but yes, the F test for Task*age is with the quadratic effect accounted for in the model.

> if I wanted to look at a quantitative variable that occurs at wave 1 (e.g., a constant within subject value across waves) to ask
> the question of "does this value at time one influence the relationship between task and age on whole brain activation?", would
> this be an appropriate way to think about the t test (assuming variables are centered)?

Yes, the coding -gltCode 1 'Task : 1*cond1 -1*cond2 age_c : -1 constant : 3' seems reasonable to me.

> -gltCode 2 'Task : 1*cond1 -1*cond2 age2 : constant : 3'\

This one is different from the one above and assesses the different age2 effect between the two conditions when constant is held at 3. And it would be probably better formulated as

-gltCode 2 'Task : 1*cond1 -1*cond2 constant : 3 age2 : '\

Gang