Hermione admitted to successfully trying out "a few simple spells" just before her first year at Hogwarts, but, in spite of underage magic being banned, she apparently never received a warning from the Improper Use of Magic Office for it. This may be because, since she had not yet attended Hogwarts as an official student, the Ministry overlooked it just as they overlooked when young Tom Riddle used magic before attending Hogwarts to move objects, help him climb down a steep cliff, etc.; and also as they overlooked when young Harry made the glass of the boa constrictor cage disappear, made his own hair grow back to normal long length after Aunt Petunia cut it, etc. Severus Snape indicated as much to Lily Evans when they were both children about Lily's own use of magic before she was a student.