Longitudinal analysis across age, when participants are all different ages from T1

Hi all!

I'm looking to do a longitudinal analysis to see how a variable changes across two time points. The thing is, from T1, my participants are all different ages (between 5 and 10 years).

I've done a regression at T1 for a cross-sectional comparision, but I would really like to utilise the longitudinal information I have. I was thinking of comparing the differences between T1 and T2 across various sets of ages (seeing as there was a 2 year gap between testing, so for example it would compare 5-7, 7-9.. etc.) but this proved to be confusing and difficult to interpret.

Any ideas? I'm sure there must be a regression analysis I could use but I haven't had any luck so far!

(Also, I am a final year psychology student who uses SPSS, so nothing too fancy please!)