“Happy Days” alum Anson Williams proved age is just a number by flexing his muscles in a shirtless mirror selfie.

The 77-year-old actor — who portrayed Potsie Weber in the beloved sitcom — took to Instagram Sunday to share a photo of himself flaunting his biceps while posing in the bathroom.

“My yearly reminder to every Boomer: NEVER let anyone diminish us because of our age,” he captioned the photo. “I’m 77. More creative, healthier, busier, more curious — and more alive than I was at 40.”

Williams credited his drive to his wife, Sharon MaHarry, who “sees the best in [him],” along with “good lovin,” “kindness” and “adventure” for his health.

He also urged fellow “boomers” to have a “little discipline,” adding, “Eat right, move, take care of yourself.”

“Mountains still worth climbing,” Williams continued. “And the absolute refusal to let anyone else decide my worth. We didn’t come this far to become invisible. We came this far to become unstoppable.”

Williams concluded his inspirational speech by saying he was “not winding down” but instead “still winning.”

“And we haven’t even played our best hand yet,” he stated.

Several of Williams’ fans gushed over him in the comments section, with one writing, “I’m 54, If a time traveler from the future showed up with this photo and told me, ‘This is you at 77,’ I would be stoked!”

“Way to go Potsie!!!” another added, referring to Williams’ “Happy Days” character. “Couldn’t resist … you’re looking great Anson!”

“Looking good and inspiring people! I am taking your message to heart. I needed a reminder,” a third chimed in.

Others praised Williams for “looking healthy and fit.”

Williams portrayed Weber on “Happy Days” for all 11 Seasons from 1974 to 1984 alongside Ron Howard as Richie Cunningham, Henry Winkler as Arthur “Fonzie” Fonzarelli and Marion Ross as Marion Cunningham.

The sitcom was Williams’ greatest hit, though he made several appearances on various shows throughout the years, including “Baywatch” and “Star Trek: Voyager.”

In recent years, Williams has stepped into a directorial role and is currently working on the off-Broadway show “Crazy Mama: A True Story of Love and Madness.”