Katharine McPhee and David Foster "aren't fighting age" in their relationship

She told PEOPLE: "Honestly, I think that we're both very similar in this way. I welcomed my 40s, and David didn't have any conversations about, like, 'Oh, I can't believe I'm 75,' or I didn't say, like, 'Oh, I can't believe I'm 40. I'm really mourning that.'

"We didn't do any of that. So, we're kind of both cut from the same cloth. It's like we sort of just accept life as it comes, and we're not fighting getting older, because it just keeps getting better."

Together, the pair have son Rennie, two, and their large age gap has led to very different views on how to parent their child.

David said: "I want to start disciplining [Rennie] and Kat's not really down with that."

Katharine replied: "No, that's not true. I just want to discipline in my own way. There's the more old-fashioned way of disciplining which involves time-outs and things like that. My take is that you can have more mindful parenting opposed to just assuming that a two or three-year-old can have time alone to reflect on what they've done poorly.

"I think the more new way of parenting is understanding that with a child, there's only so much that they can intellectually understand. Saying 'That was really bad, that was really, really bad,' gets into a shaming thing. I think disciplining is something that happens over time."

David was previously married to B.J. Cook from 1972 to 1981, and has Amy, 51, with her and Sara, 43, with his second wife Rebecca Dyer.

The couple also had Erin, 42, and Jordan, 37, together but they divorced in 1986.

He was then married to Linda Thompson from 1991 until 2005 and 'Real Housewives' star Yolanda Hadid from 2011 until 2017.

David's eldest child is 55-year-old Allison, although she was adopted as a baby and then reunited with her father in adulthood.