I have a research question that I am not sure how I should answer it. I think I may have to use growth model, but not sure! I appreciate your help!

I have 500 investors in my database, from 1996 to 2013. The investors have certain characteristics, which I aim to study. Lets say there are 3 characteristics: age, size and experience. I want to see how these characteristics have co-evolved through the time, and based on that I want to make some forecast about the market.

To do that, I set 4 time intervals, 1996, 2003, 2008 and 2013. For example, what I expect is that up to 2008, older firm had more assets, but from 2008 to 2013 this has changed and now (2013) in the market younger firms have more assets. This is why I cannot use time series. Because time series, give you an overview of the whole time span rather than each time span that I defined (1996-2003-2008-2013). To start, I run 4 regressions, using data at the end of each year and mapped Assets (Y) on Characteristics (Xi). As far as I see, the characteristics have changed. But I want to do it more professionally rather than reporting 4 regressions! What model should I use? Growth model? mixed model? I am quite confused... I use both Stata and Spss

Many many thanks!!!!