There is actually a great writeup about this titled Blond Thor - Stan Lee Wasn't Wrong.
It can be noted that Stan Lee and Jack Kirby apparently did consider the red haired, bearded design and decided against it.
In an issue of The Jack Kirby Collector, Stan Lee says
“Before starting the series, we stuffed ourselves to the gills with Norse mythology, as well as almost every other type of mythology – we love it all! But you’ve got to remember that these are legendary tales – myths – and no two versions are ever exactly the same. We changed a lot of things – for example, in most of the myths Thor has red hair, Odin has one eye, etc. But we preferred doing our own version.”
The article delves into why Thor is said to have had red hair and a beard. Their findings were surprising.
I don't want to copy/paste too much, but the TL;DR of it is that when they started looking into it, the very early texts said that he had either red or blonde hair depending on your interpretation of the material. Those early texts also waffled on whether or not he had a beard at all. It wasn't until much later (we're talking 300-400 years) that he was "definitively" red-headed. So, making him blonde and without a beard actually harkens back to what may have been his earliest depiction.
Much of the blonde design may actually stem from Steve Ditko's (a friend and co-worker of Stan Lee) earlier work on a comic called "The Hammer of Thor" for Charlton Comics. Here it was a young, blonde and beardless viking who happened to find Thor's hammer.
Donald Blake's discovery of the cane which magically transforms into Thor's hammer in Marvel's Journey Into Mystery is incredibly similar.