The law calls for a fine of $620 for minors caught driving without a special license.
Until Thursday, those as young as 14 could drive motor scooters or tiny, two-seat cars with moped engines without a license.
Traffic police officers in Milan stopped 655 drivers aboard motor scooters before they found their first culprit, a 17-year-old.
Beside the fine for not having the new license, the youth was hit with a second fine, of $120, for having a young passenger aboard, the Italian news agency ANSA said. Minors aren't allowed to transport minors as passengers.
In Naples, where drivers make other Italians look almost tame behind the wheel, about a dozen fines were handed out.
Many young people have complained that free driving courses promised for public schools were slow to get organized and that they didn't have the few hundred dollars needed for private driving schools.
But Transport Minister Pietro Lunardi ignored pleas for a delay of the new law. He said the young drivers, numbering some 700,000, had a year to obtain the new license.
But by last week, only 220,000 of them had received one.
At 18, the young road warriors are eligible for a full adult license. But adult habits aren't much better.
Grown-up motor scooter drivers often head the wrong way down streets or balance a tot or two without helmets on their scooters.
Stop signs for many Italians are something to slow down for, not stop.