The cyber attack on the company in October 2015 prompted fears thousands of people may have had their online details stolen.
The boy told magistrates: "I didn't think of the consequences at the time. I was just showing off to my mates."
"It was a passion, not any more. I won't let it happen again. I have grown up," he added.
His solicitor Chris Brown said he had played a small part in the scam, adding his behaviour had been that of an immature 16-year-old.
The charges against the boy also included attacks on other websites, including the universities of Manchester and Cambridge.
The boy will be sentenced for breaches under the Computer Misuse Act on 13 December. The magistrate said although further reports were needed, they were minded to spare the teenager jail.
Six other people were arrested in connection with the attack.