"She's 21 years old and her career is skyrocketing right now and it's tough to sustain a relationship, especially with somebody who's so much older than her," Derek, 24, told PEOPLE Thursday at Diddy's 40th birthday bash at New York's Plaza Hotel.
As for how Julianne, 21, and Wicks, 30, are each handling the split, Derek says, "They're both good. He's a great guy – he's cool."
And the ex-couple will spend time together when their schedules allow. "We'll all still hang out," says Derek. "We're all very close. And [the split] is amicable. They're still really great friends, and that's the way to do it."
Derek should know – he speaks from experience when it comes to remaining on good terms with an ex. His date at the party Thursday was former Dancing partner and on-again, off-again girlfriend Shannon Elizabeth, 36. Asked whether she and Derek might one day rekindle their romance, she was not optimistic.
"We're in different places in our lives," she said. "If he gets past the place he's in now to a new place, then maybe. I'm a little bit older … He goes out partying every night. He doesn't like to stay home alone."
And now that Derek's sister available as a party companion, he has one less reason to.