Is the statement "I pledge to myself" to stop watching pornography and masturbating for a month considered a vow (nadhr) or an oath (yameen)? What is the expiation (kaffarah) for not fulfilling it, knowing that the person who said it is 17 years old and was unaware of the rulings of vows and oaths at the time of uttering it?

One must keep away from what Allah has forbidden, protect oneself from sins, be mindful of Allah in private and in public, and not listen to the whispers of Satan. The phrases "I pledge to myself" or "it is a commitment upon me" are neither vows (nadhr) nor oaths (yamin), so there is no expiation for violating them. However, fulfilling them is obligatory, and breaking them is a bad deed.