Sarah Michelle Gellar, 48, looks youthful as she kisses husband Freddie Prinze Jr on his milestone birthday

Sarah Michelle Gellar showcased her youthful glow as she celebrated her husband Freddie Prinze Jr.'s milestone birthday over the weekend.

On Sunday evening, she helped her husband ring in his 50th birthday by throwing a party and shared two photo booth snapshots of them together for a tribute post on Instagram.

'Happy Milestone Bday @realfreddieprinze,' the Buffy the Vampire Slayer star, 48, wrote in her caption. 

'Age is simply the number of years the world has gotten to enjoy you,' she continued. 'And no one has enjoyed you, more than me.'

The couple, who have been married for over two decades and are now cemented in status as one of Hollywood's longest lasting love stories, looked giddy as they posed together.

One snapshot showed the pair both puckering their lips as they made fun expressions while the other saw the longtime couple packing on the PDA.

Prinze Jr. had a big smile on his face as they wrapped their arms around each other and his wife planted a kiss on his face. 

The pair, who first met on the set of the 1997 horror film I Know What You Did Last Summer, seemed barely touched by time in the adorable photos. 

For Prinze Jr.'s milestone birthday celebration, he wore a light brown, cashmere button-down over a taupe T-shirt.

Gellar turned up the glamour in a sequined minidress and her blonde tresses styled into messy beach waves. 

The longtime couple, who will be celebrating their 24th wedding anniversary in September, were friends first and then dated for two years before going on to tie the knot in 2002.

Prinze Jr. recalled the early days of their relationship at the premiere of the reboot of I Know What You Did Last Summer last year.

In an interview with Access Hollywood, he said: 'We were friends for a good couple of years before any sort of date happened. 

'We met on that movie and became really close, really good friends,' he explained. 

'We lived ten minutes away from each other, so we made it a point to maintain the friendship and it turned into something else.'

He also previously revealed that their first date was accidental after a group dinner unexpectedly became a dinner for two.

'We were supposed to go out with a group of people, and everybody else bailed… so it was just the two of us,' Prinze Jr. said on The Howard Stern Show. 

They dated for a few years before Freddie popped the question in 2001. 

The couple has never revealed the details of the proposal with Gellar previously saying: 'I didn't want a big public proposal. I wanted something private.' 

After getting married, the pair also worked together on a number of projects including the live-action Scooby-Doo movie, which was released in 2002, as well as the 2004 sequel, Scooby-Doo: Monsters Unleashed. 

The pair also both voice acted in the 2006 animated film Happily N'Ever After. 

A few years later, they expanded their family, welcoming their first child in 2009 and their second in 2012. 

The couple are parents to daughter Charlotte Grace, 16, and son Rocky James, 13.