Mr Key will spend the day at Parliament before flying to Auckland to have dinner with his wife Bronagh and children Stephie and Max.
Asked about his milestone birthday, Mr Key said he felt lucky.
"You're as young as you feel, aren't you, so I feel pretty lucky," he said.
"I'm old for an investment banker, but I'm no longer one of those, and I'm young for a prime minister, so I'm not too stressed about it."
Mr Key said he still hoped to be in Parliament in five years time.
"I'm enjoying it, it's gone a lot quicker than I thought ... I don't know if it's just the fact that New Zealand's faced so many issues in the three years I've been prime minister, but it's sort of flashed by."
Mr Key is expecting a birthday call tonight from Russian President Dmitry Medvedev.
"Apparently he has a habit of ringing, dare I say it, leaders that he likes," Mr Key said.
"He sent me a silver tea service, which was very nice of him, as presents. So I've got that in my office and he's going to give me a ring tonight."
Labour leader Phil Goff also sent Mr Key his best wishes for the milestone.
"Wish him well on his birthday and hope that he can get some time to spend with his family."
Finance Minister Bill English quipped outside the National Party caucus this morning that he was being held up from eating birthday cake.
