Late 08's & 2009's were the first to get newly designed inner fenders. However, that doesn't nessecarily mean improved. I believe there are late 08's and early 09's with fender tears, cracks, or buldges.
2010's got a new frame, but still apparently are susceptible to cracks.
I wouldn't base your purchase year based on fenders, personally. Just make sure the one you buy doesn't have them when you drive it off the lot.