I have winter tires that are very little used (perhaps 5k to 10k kilometres, or equivalently roughly 3k to 6k miles) but almost 10 years old (produced in 2008 week 21). I cannot use them in winter any longer since the rubber has lost its softness and has become pretty slippery. However, the tires still look great (almost like new) and I am wondering if they could be used in the summer? 

Since the rubber is harder now than it was when the tires were new, perhaps they are actually better suited for summer use now than before. It would be a pity to just throw them out seeing that they have almost all of the original tread and no other visually detectable faults.