Katharine McPhee and David Foster “aren’t fighting age” in their relationship. The 40-year-old singer has been married to record producer David, 75, since 2019 and she has insisted that they both “welcomed” their recent milestone birthdays as opposed to anything else. 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 clot…