Is it permissible for an eighteen-year-old young man to propose to a girl he loves without referring to his parents or a sheikh?

The only permissible way for a loving young man and woman to have a relationship is through a legitimate marriage (zawaj shar'i), which is only valid with conditions and pillars, including a guardian (wali) and witnesses. It is permissible for a man to propose to a woman, informing her of his desire to marry, provided there is no fear of temptation (fitna), but it is more virtuous for the proposal to be made through the guardians.