Inside Pharrell's 'Happy' Birthday Bash!

ET was the only show allowed inside Pharrell Williams' 41st birthday party!

ET was the only show allowed inside Pharrell Williams' birthday party, where the record producer and singer-songwriter proved to Brooke Anderson that you're never too old to feel like a kid again.

Nickelodeon threw the SpongeBob SquarePants-themed soiree for Pharrell, marking his 41st birthday.

VIDEO: Pharrell Talks Grammy Hat & Performing for Beatles

The Happy singer remains a big fan of cartoons, which could be the key to his youthful appearance. Brooke pointed out that it's been nearly 10 years since his hit single Drop It Like It's Hot with Snoop Dogg and Pharrell still looks the same as he did in the video.

While he's been in the industry for well over a decade, Pharrell is more sought after than ever these days with his new gig coaching on The Voice this fall.

"I signed up for two reasons," said Pharrell. "To find new talent, but also I was hoping the people at home would see the coaching that I'm going to do and be able to take something from it."

Pharrell's album G.I.R.L. is out now.