Ships, like all physical assets, depreciate over time. They generally have an economic life of about 25 years. Although in recent years this has been significantly less time due to shipping rates being so low owners stopped the bleeding by scrapping ships.
During the last 2 quarters however, the values of many classes of ships has increased due to S&P pressures.
With that said, DRYS oldest ships were built in 2000 and the company will need to make a decision by 2020. Sulphur emissions kick in then and it probably won't make economic sense to upgrade the ships at that point.