how to control foe age-wave1 when running a mixed model analysis

Hi guys,

I am running a linear mixed model in SPSS. My research question is: Does a reward-related personality trait at baseline predict weight gain in adolescence?

That's my design:
longitudinal data (w1,w2,w3,w4,w5)
dependent variable: bmi measured at the 5 time points
factors and covariates: gender and the reward-related personality trait

My issue: Some participants were 14 at baseline. Other were 15, 16 and 17 at baseline. I noticed that there is an interaction between age at baseline and weight gain. However, I am not interested in this interaction and I would like to control for age.

How can I do that in SPSS? Many thanks in advance.