To paraphrase cinema’s most infamous lech, the thing about pop music is that even as we get older, the stars stay the same age. That’s generally the promise of Jingle Ball, iHeartRadio’s traveling circus of pop radio hitmakers, which touched down Monday night at Capital One Arena. Amid a red-and-green light show that kept Pepco paid, the three-hour program showcased pop’s youth movement, along with a few acts whose hits already feel like oldies.