Wait, what? They just took a Dalek TARDIS to get home at the end of "the Chase", had a happy montage of ordinary Earth life and that was it. There was nothing onscreen to imbue them with any special powers.
Nope. But at the end of a RTD-penned episode of Sarah Jane Adventures, (Death of the Doctor) Sarah is explaining that she's looked up several of the Doctor's companions, and she specifically calls out Barbara and Ian as not having aged a day in 40 years. So, whatever the in-universe reason, it appears to be something that has happened.
EDIT: Here's the exact quote:
"There’s a woman called Tegan in Australia, fighting for Aboriginal rights. There’s Ben and Polly, in India, running an orphanage there. There was Harry. Oh, I loved Harry. He was a doctor, he did such good work with vaccines. He saved thousands of lives. There was a Dorothy something. She runs that company, A Charitable Earth. She’s ...raised billions. And this couple in Cambridge. Both professors. Ian and Barbara Chesterton. Rumour has it, they’ve never aged. Not since the sixties. I wonder… echoes of the Doctor, all over the world. With friends like us, he’s never going to die, is he?"