I'm going to guess it was Kirby with Starlin being the less prolific artist that I recall recognizing.

Tough to recall for sure, and I was able to tell the difference among art styles right off the bat even if I couldn't actually name an artist yet, but I believe it was probably Neal Adams or Kurt Schaffenberger. Probably Adams, by a nose.

If I had started reading comics a few months later, it likely would have been Jose Luis Garcia-Lopez.

Michael Sekowsky --- with the early days of the Justice League
Jack Kirby --- with the Fantastic Four, and then the Avengers
George Perez --- with many titles
José Luis García-López --- I can recognize his work anywhere
John Byrne --- his work on Superman


These are my top five artists that I can think on the top of my head.

Neal Adams on his early 70's Superman covers or Mike Sekowsky on Wonder Woman (whose work i compared to a pinball machine's artwork from the local swimming pool)

In my early reading, it was probably Kirby FFs and Ditko Spider-Man.
When I became a regular reader/collector in my teans I could recognize most artist.
Especially Barry Smith and the Marvel Bullpen.


Wait, I take that all back.
The first artist I could recognize was Mad's Don Martin.

A few days ago in another thread I'd mentioned that inker Jack Abel was probably the first artist whose work I could recognize. This was back when he took over as the main inker for the Weisinger books (when George Klein, one of the group of regular DC freelancers who asked for health benefits, etc. was so ignominiously told to go elsewhere***).  At the time DC didn't usually post credits in-story, at least not for the Weisinger books, but I could recognize Abel's work whether he was inking Swan or Win Mortimer (two very different pencilers).  He had such a thin line; and as I mentioned in my other post his face work was very distinctive--especially the female character's eyelashes.

Over at Marvel I always recognized Vince Colletta's work no matter who he inked. In my early comic reading days I came across his inks on John Buscema, Don Heck and Gene Colan (that Medusa story in Marvel Superheroes). I could recognize Colletta's work because of how he texturized hair, adding lines plus a tiny bit of stipling; the full lips he gave many of his characters; and again, the females' lush eyelashes.  I loved his '60s work. 

In terms of pencilers, well, has to be Kurt Schaffenberger. In the Silver Age I'd read a few issues of Lois Lane and then when I got #78 I remember thinking how different Lois looked on the cover. It was because Curt Swan drew that particular cover and not Kurt (who usually drew both interiors and covers of the LL book up until that time). I could tell #78's cover was not Kurt's work, even if I didn't know the names of the artists back then. 



***He went over to Marvel and became John Buscema's inker on the Avengers; and was, IMO, Buscema's best inker 

It became a very popular style in mainland Europe, up until the '70s, yes, particularly in Belgium and France. But was less popular in the UK, although the Tintin books are very popular over here, of course, but they were the exception, rather than the rule. The ligne claire style was actually developed by Hergé in 1934 in The Blue Lotus, and was then taken up by many bande dessinée artists, such as Edgar P. Jacobs, Jacques Martin, Jef Nys, and Albert Uderzo (who accentuated the cartoonish elements of it in the Asterix books) among others.

I don't know how popular the Where's Wally? books are in the States (they were a bit of a phenomenon over here in the 1990s), but they utilise the ligne claire style too.