As for how the longtime pals – who will kick off their Future Now tour in Atlanta on June 29 – navigated their successful transitions from their Disney days?
"I attribute it to just learning. When you start working at a young age, the only way you can transition is to allow yourself to grow with your fans and not rush or delay it – just live it honestly. I had to learn that the hard way," says Lovato.
The "Stone Cold" singer had a nervous breakdown in 2010 before facing her struggles with addiction, body dysmorphia and bipolar disorder; today, she champions body-positivity and mental health care reform.
If she could give her younger self advice, "I would say, 'You're beautiful just the way you are. You don't need to change for anybody.' That was definitely something that weighed on me when I was younger."
Happier and healthier today, Lovato, 23, has a solid support system – which includes her tour-mate.
"We've grown up together, and Nick is somebody that I can rely on, whether it's in the middle of the night or the middle of the day – he's consistently been there for me," she says.