There's an age difference of 64 years, but Hugh Hefner and his son Marston share a birth date, April 9.

And the pair decided to celebrate together this year, with a joint bash at the Palms Hotel in Vegas honouring the Playboy founder turning 85 and the philosophy student becoming 21.

Marston was accompanied by a blonde playmate whilst Hugh had fiancée Crystal Harris, 24, on his arm.

And despite the age he was celebrating, Hugh spoke of how youthful he felt.

"I'm a very lucky cat and I know it, age is just a number if you're healthy," he told Radaronline at the bash.

Marston's mother is Hugh's second wife, Kimberley Conrad, 48.

Photo: © Rex