Carbon fiber bikes crack too. The thing with aluminum frames is that they are supposed to be more prone to the material fatiguing as they get older. I have had several aluminum road and mountain bikes and have never had a problem. If you have looked the frame over really well and not found any signs of stressing or cracking then it is fine.
Several years ago I bought a ten year old aluminum trek, rode it for about 10,000 miles and then sold it on to someone else who is still riding it.