I have my BB set to "keep email for 15 days". However, there's mail from longer than 15 days on my BB.
The last time I noticed this, I deleted ALL of my old email, and made sure the setting was on 15 days. It's been about 23 days, and all of my email is there.
I used to have that problem on my Pearl 8120 and then one day I tried installing OS 4.5 in the process of which I wiped my handheld and stuff...then I eventually switched back to 4.3 and it started working.