Paging Dr. Mark Sloan.

As the legendary Dick Van Dyke reaches the milestone of his 100th birthday this Saturday, one of his co-stars from “Diagnosis: Murder” reflects on a missed opportunity during their time on the show. Charlie Schlatter, who shared the screen with Van Dyke for six seasons of the beloved mystery medical crime drama, recently shared a nostalgic regret with The Post: he wishes he had more personal photos with the iconic actor.

Despite spending years on set together, Schlatter, now 59, was surprised to realize just how few snapshots he has of those memorable days. “It’s so funny,” he mused, “because so many people assume I must have tons of pictures with Dick Van Dyke.” Yet, he can count the personal photos on one hand. “I literally only have three or four personal photos,” he admitted, recalling that one was taken at a wrap party and another in the hair and makeup trailer.

Reflecting on this lack of documentation, Schlatter noted that at the time, it simply wasn’t something that crossed his mind. The oversight seems understandable, considering the focus was likely on the work at hand rather than preserving memories for posterity.

The “Bright Lights, Big City” star went on to explain that it wasn’t something he thought much about at the time.

“It’s not like today, where we have [our phones],” Schlatter noted. “That’s my biggest regret. I wish I had more. You were always just sitting around with him, and you never thought to get up off your ass and get a picture.”

Schlatter joined the popular CBS series during Season 3 in 1995 as Dr. Jesse Travis, the eager young resident under Van Dyke’s Dr. Mark Sloan.

He remained on the program, which also starred Van Dyke’s 74-year-old son, Barry Van Dyke, until it came to an end after eight seasons in 2001. Schlatter credits the “Mary Poppins” star with being the one to help him land the role of Dr. Travis in the first place.

“I call my home the house that Dick built, because he wanted me on that show,” the “18 Again!” actor told The Post. “It’s funny, the character’s name was Jesse Travis, and I think he was described as this six-foot-four whatever.

“I remember being at the audition, me at five foot four, being with all these surf guys,” Schlatter continued. “I think there was something about my presence that made him laugh. I think he even says in one of his books that they needed a little Michael J. Fox guy.

“So he really called the shot on that casting, and God bless him, he’s the one who made that happen.”

Now, as the Danville, Illinois, native turns 100 and more than three-decades after the pair first met on the set of “Diagnosis: Murder,” Schlatter has revealed what he thinks it is that has helped the “Chitty Chitty Bang Bang” star reach triple digits.

“He’s taking care of himself physically. He always did,” the “Delinquents” star shared. “When I knew him, it was always an hour at the gym; he was always swimming because, if not, atrophy would set in.”

Van Dyke’s past as a premier slapstick comedy performer, however, was always something that the “Bye Bye Birdie” star took into consideration amid his disciplined and daily workouts.

“He said, ‘You know, Charlie, I fell over that Ottoman way too many times,’” Schlatter recalled regarding Van Dyke’s signature bit on “The Dick Van Dyke Show.”

“He’s paying for it,” the “Ferris Bueller” TV star added. “This was almost 30 years ago, when he was telling me that.”

But staying active, Schlatter acknowledged, is only part of the secret to his former co-star’s longevity.

“The other thing is just his joy of life,” the “Police Academy” actor said. “I think he really just enjoys living, and he enjoys people.

“He has a genuine curiosity about others and about life,” Schlatter continued. “He loves music, and he loves dancing, and he loves to be surrounded by people who are like-minded.”

As for whether Van Dyke ever shared the “key” to his long life with Schlatter, the “Heartbreak Hotel” star explained that it wasn’t something that could be spelled out.

“He was never the professor in the sense that he would sit you down and say, ‘This is the key to longevity,’” Schlatter explained. “Every day that you were at his side, you were at school, and you’re an idiot if you didn’t learn something from that.

“If you walk away from seven years of working with Dick Van Dyke and you don’t have something good from it, there’s really something wrong with you.”

Working with the acting icon on “Diagnosis: Murder” will be one experience he’ll never forget.

“As my grandfather said, ‘Charlie, you’re working with the master now,’” the “Flash” voice actor said. “Because he knew he was just the master of comedy.

“I mean, look at him. He’s the master of life,” Schlatter concluded. “That guy has hacked the s–t out of life.”