Exactly
The person making that may have not had as much involvement with the earlier Toy Storys. What’s in the final script is that he’s 6. No arument there. Andy’s age in Toy Story 2 is thought to be 8. This is from an official source (though, like here, they do misinterpret things). Another reason for that is that he’d have to have settled in a little from the last movie, and he can’t be older than 8, because that’s how old he is in the beginning of Toy Story 2.
Not to mention Molly hasn’t really aged.
I really like that concept art picture. I love pencil sketches. However, I don’t know it really says what Andy’s age is in a certain year.
It will never make sense no matter how much we read into it and obsess. So I think I’ll stop obsessing over this detail. 
Pixar’s being so mysterious hiding Andy’s age and locations, they’re doing it to bother fans, I knows it!
They do it with a lot of stuff. I like that technique, because then you can make it all how you like it.
This is Fred. Fred lives (insert your state here.) Fred is (insert your age here.)
I like it. The characters are nore relateable that way, because you can make them your own and imagine little scenarios and everything! <3
My personal belif is that Andy was 5 turning 6 in the original (set in 1995),8 in Toy Story 2 (set in 1997 not 99), and finaly 18 in Toy Story 3 (set in 2007)even though that would go against Mr.Potato Head saying “the kids 17 years old,” but I just like to think he’s wrong. 