Alan Young is still alive, having been born in 1919. He’s 94.
(otherwise known as W-w-w-willllburrr, owner of Mr Ed)
Wikipedia says he had his first radio show, The Alan Young Show, in 1944.
Back before Glynis Johns was the suffragette mom in Mary Poppins, she got plenty of pre-1958 TV work – on The Errol Flynn Theatre, and Studio One In Hollywood, and Schlitz Playhouse, and as per the OP even a spot on Sid Caesar’s Your Show Of Shows, which ain’t bad for someone who made all those movies as a teen in the '30s.
Another one who has failed to make the list for the worst reason is Mary Grace Canfield. She played “Ralph” the handman on Green Acres. First TV credit in 1954.
Passed away Saturday.
Aw, man. RIP, Ralph. 
I was amazed to learn that she was already in her 40s when GA was being filmed. I always thought she was at least ten years younger! :eek:
Well, hit me on the side of the head and call me stupid! Turns out Max Baer, Jr (Jethro Bodine on The Beverly Hillbillies) is still around too at age 76, though he hasn’t done anything in TV or movies for quite some time.
And Donna Douglas (Ellie May) at 80.
… As was noted above. 
Whoops! Turns out Max didn’t start acting professionally until 1960, according to Wiki. Bummer!
D’oh! Brain blip. :smack:
Before he was Baretta – but after he was one of The Little Rascals – Robert Blake was a twentysomething during the Golden Age of TV, playing an Apache warrior on Broken Arrow in '56 after popping in for an episode of The Cisco Kid but before filming an episode of The Roy Rogers Show; the guy who played Rain Cloud on an Adventures of Wild Bill Hickok episode in '52 had plenty of work back when.
he was busy with his cyphering before then.
Don Rickles has a few IMDB credits from the mid 50’s
Yep, the boy took to cyphering like a mule takes to millet! 
Piper Laurie has pre-'58 acting credits on Playhouse 90, General Electric Theater, and every other show from Robert Montgomery Presents to Studio One In Hollywood, plus a couple of TV movies and appearances on The Ed Sullivan Show and This Is Your Life.
The youngest celebrity in age still around from early TV has to be Ron Howard, even if The Andy Griffith show started in 1960.
Still, Keith Thibodeaux – who played Johnny Jason on The Andy Griffith Show – got his start pre-'58, as Little Ricky on I Love Lucy
I can’t believe that I forgot Clu Gulager. He was in Playhouse 90 in 1959, he had minor TV roles from 1955 forward and his latest credit is an acting role in 2012.
Reference:
Diahann Carroll was a '50s-game-show contestant on Chance Of A Lifetime before she started performing on The Red Skelton Hour and The Ed Sullivan Show and acting in a Ronald-Reagan-hosted episode of General Electric Theater opposite…
…well, Harry Belafonte, who (a) had been on television since the '40s, and who (b) had just played a leading role on Front Row Center with…
…well, Marge Champion, who has television acting credits dating back to '49, when she was on The Philco-Goodyear Television Playhouse with…
…well, Conrad Janis, who like the rest of 'em racked up plenty of pre-'58 TV credits. Why, in '52 he was in a television drama with…
…well, Sidney Poitier, who I mentioned on the previous page, so, yeah.
He was also a paperboy in the classic Treasure of the Sierra Madre (with Humprey Bogart).
There’s always Polly Bergen, who I mention not because of her pre-'58 appearances on The Ed Sullivan Show and The Jonathan Winters Show and The Jimmy Durante Show and The George Jessel Show and The Alan Young Show and The Jackie Gleason Show and Max Liebman Presents: The Maurice Chevalier Show, but for – well, just hosting The Polly Bergen Show starting in '57.
You know, after acting on Playhouse 90 and General Electric Theater and et cetera.