Born in 1955 so like 29 in 1984. Welker was born in 1946. Cullen born in 1941.
I meant to do this yesterday, but to the best of my really quick research, these are the OG cast members who are still alive:
Alan Oppenheimer (Beachcomber, Breakdown, Seaspray, Warpath) b. 1930
Michael Bell (Prowl, Sideswipe, Scrapper, Bombshell, Swoop) b. 1938
Peter Cullen (Optimus, Ironhide) b. 1941
Terry McGovern (Wildrider) b. 1942
Marlene Aragon (Elita-1) b. 1943
Neil Ross (Bonecrusher, Slag, Slingshot, Springer) b. 1944
Arthur Burghardt (Devastator) b. 1945
Frank Welker (like half the cast. Seriously. But mostly Megatron, Soundwave, Skywarp, Rumble/Frenzy,
Sludge, Mirage, Trailbreaker, Wheelie) b. 1946
Laurie Faso (Divebomb, Rampage, Skydive, Orion Pax) b. 1946
Arlene Banas (Carly) b. 1946
Susan Blu (Arcee, Marissa Faireborn) b. 1948
Steve Bulen (Strafe, Sureshot, Peacemaker) b. 1949
Dan Gilvezan (Bumblebee, Hot Spot, Outback) b. 1950
Gregg Berger (Grimlock, Long Haul, Skyfire) b. 1950
Michael McConnohie (Tracks, Cosmos) b. 1951
Danny Mann (Freeway, Lightspeed, Cloudraker, Spoilsport) b. 1951
Michael Horton (Chip Chase) b. 1952
Jim Cummings (Afterburner, Rippersnapper, a Sharkticon once) b. 1952
Beau Weaver (Octane) b. 1952
John Moschitta (Blurr, Punch/Counterpunch) b. 1954
Townsend Coleman (Rewind) b. 1954
Hal Rayle (Pipes, Shrapnel ,Snarl) b. 1955
Corey Burton (Spike, Shockwave, Brawn, Sunstreaker, Vector Sigma) b. 1955
Charlie Adler (Silverbolt, Triggerhappy) b. 1956
Rob Paulson (Air Raid, Fastlane, Slingshot) b. 1956
Paul Eiding (Perceptor) b. 1957
Maurice LaMarche (Six-Gun) b. 1958
Brad Garrett (Trypticon) b. 1960
David Mendenhall (Daniel) b. 1971
There are also a handful of actors who have no credited date of birth on TFWiki
Michael Chain (Hoist, Powerglide, Red Alert, Skids (once))
David Workman (Nosecone, Sinnertwin)
Joe Lahey (Razorclaw)
So it turns out that there are a lot more of the cast alive than I thought, though it's probably still more like 60/40 in terms of those who have passed vs those who haven't.
Also Corey Burton isn't the youngest. He's not even in the top 5 youngest on the show unless you discount David Mendenhall because he never voiced a bot and Maurice LaMarche because he did one episode of work then quit the show because he couldn't stand working for the voice director.