PROC MIXED - IV which Level

[H]

I am running a PROC MIXED for repeated values on a test (taken quarterly). Subjects have from 3-38 repeated test values. Age should be a predictor of test score. Age seems like a group-level (random effect) variable and I am inclined to put it in level-2, but it changes across time, so it isn't a constant per se. I wanted to get confirmation to place it in the second level of the model.

Also, the quarterly test can be missed and isn't always perfectly/evenly spaced. Do you have any recommended covariance structure or should this be based on AICc and context?

Thanks,

H