Since I primarily associate Neal Adams with very fresh/modern looking art in the 1970s, it took a while for me to accept the idea that he came in at the tail end of the Silver Age.

I only learned recently, like about a decade ago, that Irv Novick had been working continuously in comics since the late 1930s; before that, I'd always thought he was one of the 'younger' generation that got its start in the 1960s.
Another creator I was surprised to learn was actually older than I am is Lee Weeks. Even though he got his start in the late 1980s, I only became familiar with his work after my long hiatus from comics, so sometime in mid- to late '00s, so I thought he was one of the newer generation of creators.

I was a little surprised when I recently read that Wally Wood was only 54 when he died. I guess it seemed he was "always there" as I got into comics and had several phases in his career.

I remember being surprised when I picked up an early-50s comic and finding a Gene Colan story in it. I thought he'd come into the business later than that.

I've never really had that moment, though I had not realized, at the time, how young Colleen Doran was, when she started A Distant Soil, until I read the section about her in The Comic Book Rebels. She spoke of being underaged and getting hit on at conventions and fending off editors and such. There was one story of finding out she was booked into an editor or publisher's hotel room, at a convention and she slept in the hotel lobby, instead. Pissed me off just reading it.

Age of professionals doesn't usually surprise me. I was 17 when I started the Navy ROTC program, because my birthday is in November and my freshman year in college started in August. My parents actually had to sign a waiver to grant permission for me to enter the program. I was 21, when I received my commission and didn't turn 22, until I was halfway through the program, at Supply Corps School, following my college graduation. I was a very young ensign; in maturity, as well as years. Did a lot of growing up in those 4 years, to where I was a very old 25, when I started working for Barnes & Noble. I did have a relationship with a co-worker that did surprise the pair of us, when we discovered what our actual age difference was. I was 30 and she was 46; but, she thought I was several years older and I thought she was several years younger, which just shows that chronology has less to do with things than experiences.

He did an interview with Comics Scene magazine, before Youngblood came out.  It explains a lot!  He actually wanted to drop out of high school to try to break into comics, but his parents forced him to finish school first.  That wasn't the revealing part to me; it was a section where he is talking about some of the characters and he mentions his "big guy", Brama, was based on an idea he had, inspired by a set of triplets he knew.  He said he asked how that happened and was told that it was one ovum that split into 3 (which is far rarer than more than one ova being fertilized or even two ova, with one splitting into two, but the other not).  He continued, "So, my warped mind said, what would happen if the egg didn't split?  Would there be a giant baby?"  I read that passage 3 times, dumbfounded.  Obviously, he either skipped biology or was too busy doodling in class.  I mentally retorted, "Normal birth!".  He actually thought triplets came from a giant egg that splits into three and/or that a physically large person was the result of the size of the egg/zygote, not the DNA.  That pretty much told me all I needed to know about him and the level of thought that went into his comics.  Still didn't prepare me for his concept of WW2, with German soldiers wearing silver helmets! (In one of those Youngblood comics, in a flashback).

In the years since, I have read things on-line from people that make him sound like Einstein, by comparison; so, I cut him more slack, these days.

He had a passion for comics at an early age but he also needed the money to help his family financially because his father was in and out of hospitals with brain tumors. The father was actually in a 8 months long coma. There are  human beings behind these comic books. 

Zeck's earliest credits are only two years after Starlin's, but Mike spent a few years working mostly at Charlton. If you lived in an area that didn't get many Charlton books, it would have looked like Zeck was much younger.