As long as I can state with 95% confidence using ANCOVA that my slopes are statistically different due to location or in this case ID I am very happy with the original model. I have included the age*age, age*age*age, and age*age*age*age just to be sure.
I think that your conclusion is sound and defensible even with the slight pattern in the residual plot.
The new analysis with the higher orders should include ID alone and crossed with each of the Age terms. (Sorry I did not make that point clear before now.)
I included ID in this last model and received the error "Warning: Ill-conditioned regression problem" and the following output.
Thanks again for you help,
I think that result is because not all of the ID levels have enough data to fit all the higher order terms.
Right. I think that the best you can do is to use the reduced model (Age, ID, Age*ID) and the tests that the provide. (We already covered that option.)