I know using 30 as the average age for the first year of FA is just a quick heuristic that you used (understandable that you would too given the grossness in figuring out service time stuff for each player) but did you find that 30 was in fact the average age?
Even if you did, I think there's a slight problem with that. The players who reach free agency earlier are, by in large, the better prospects. So having a few of their prime years in the sample might skew things significantly.
I apologize if you answered these questions in the article as I skimmed it really quick. That was just an initial thought from reading about a paragraph and looking at the results.
Cool/important article topic though.