Especially with a 12-year old car, low freon would be the most likely culprit. Studies have shown that normal leakage rates are about 6% per year, so after 10 years or so, the freon will likely need a recharge. My '99 E430 hit its time earlier this summer.
I recommend recharging first, since it's simple and cheap, and then if there are still problems, diagnose further.

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