Appearing on The Late Show with David Letterman, the former Alias actress said that while she is happy to be getting older, Affleck is less pleased with the ageing process.
"I like being 40. It's interesting that you ask that, because my husband turns 40 tomorrow. I get back [to California] around 3, and he's like, 'You're not going to be here in the morning to make my breakfast?'" Garner shared.
"I don't know that he's as thrilled with the idea of 40 as I am. I really like it. I had no problem with it. I feel like, what more could I want? I can't speak for him, but it doesn't look like he's as happy as I am about turning 40."
Letterman playfully questioned why Affleck has a problem with another birthday, asking: "What's the problem? Is it low T?"
Garner responded: "Low T as in low testosterone? Are you asking about Ben Affleck's testosterone level? Really, David! He will thump you. Have you seen my husband lately? He is walking testosterone."
The star recently revealed that Affleck wants a fourth child but she is not prepared to expand their family any further.
Garner gave birth to the couple's third child, a son called Samuel, in February.