What is the ruling on one who sees his mother's body at the age of 18? And is this forbidden?

It is not permissible for a son to see any part of his mother's body except what is permissible for other mahram relatives to see, and she must cover what is beyond that. If there is a fear of fitna (temptation/strife), then she must cover her entire body.

The reason for such looking might be a neglect of proper upbringing. Children must be raised to seek permission, lower their gaze, and exhibit modesty, as stated in the verse: "O you who have believed, let those whom your right hands possess and those among you who have not reached puberty ask permission of you three times," and His saying: "And when the children among you reach puberty, let them ask permission as those before them have asked permission."

A child who looks at his mother or mahram relatives with suspicion is considered morally corrupt, and his family must advise him, and his father must discipline him.