The Opium Wars absolutely were a turning point in Chinese history. Before these wars, the Chinese were to some degree in the "driver's seat" in terms of trade with the West. Westerners were confined to their colony in Guangzhou. China had things the West wanted while the West had little that China wanted. China was in control.
But then the Opium Wars came and changed all of that. Now, China was open to opium trading and the balance of trade shifted. Now, the Western powers were able to impose unequal treaties on the Chinese and have a great degree of power over China.
These wars made China go from being in control of the trade with the West to being controlled by the West. Therefore they were definitely a turning point in Chinese history.