It jarred upon him, this habit of his mother's of referring to herself in the third person

It jarred upon him, this habit of his mother’s of referring to herself in the third person, as if she were addressing a baby, instead of a young man of ten who had taken the spelling and dictation prize last term on his head.
(a) ‘It’ here refers to what?
(b) How old is the person mentioned in the extract?
© What does ‘jarred upon’ mean?

(a) It’here refers to the habit of his mother of referring to herself in the third person.
(b) The person mentioned in the extract is ten years old.
© ‘Jarred upon’ means to have an unpleasant and disturbing effect.