I've looked it up and studied the usage a little, and that just isn't true. Occasionally, you will find definitions such as "lack of belief in a god", but they are rare. Usually the definition contains "denial of belief in gods" or "rejection of belief in gods". In any case, dictionary definitions do tend to be broader than people generally realize.
They have to be broader. They have to define it for every sense it's used in. I haven't found "lack of belief in God or gods" to be rare, but even if it was, the dictionary's not the be-all-end-all of word meanings.
For example, Merriam-Webster gives one word sense definition of "bachelor" as "an unmarried man". But that is just false. The Pope and priests are not bachelors, because they are not eligible for marriage. So the definition really needs to be tightened to mention eligibility to be more accurate. Dictionaries contain a lot of errors of that sort.
They don't contain errors (except for typos and things). You may use the word a little differently than the dictionary indicates, but that's not an error on the dictionary's part. As for your example, I would call the pope and priests "bachelors". Why not? I don't think you need to be eligible for marriage to be a bachelor. When you have a bachelor pad as a young, unmarried man, does that imply that you're going to get married? No. It implies that you're single. That's all.
But we don't need to quibble over this. Word meanings are empirical matters. Make an informal survey on the matter and pass it around to people you know. Put the question to anyone: "Would you consider a baby to be an atheist because it lacks belief in God?" I predict that most people will answer in the negative. Ask the same question of atheists, and you will get a larger percentage of people like Panda who answer positively. That is because this is such an ideological issue.
I think it's more because of the connotations of the word "atheist". I think many people have a negative view of the word to begin with, and so that will affect their usage of it. Many people have many misconceptions about atheism, and that will also affect their usage of it. So, obviously, you'll get more atheists saying "yes" than non-atheists. However, you'll get more homosexuals voting "No" on Prop 8 than heterosexuals. Sometimes it's not just a bias influencing you, it's a lack of bias the other way helping you see it differently.
I wouldn't call babies either atheists or agnostics. They simply don't know what gods are and have no opinion at all about them. Agnostics are people who deny that it is possible to know whether God or gods exist, or else they are in a state of mind where they are undecided. Babies are not undecided, because they have nothing to make a decision about yet.
OK, well, I disagree. I think if you don't believe in a god, you're an atheist.
A lot of atheists object to calling children of Christians "Christian" on the grounds that children don't really make a serious decision about religious faith until they mature into adulthood. Until then, they are undergoing an extended period of religious indoctrination. I see the point of the argument, but I would still call children who believe in Christian doctrine as they understand it "Christians".
That's weird. I see that as contradictory. If you feel "Christian" is an acceptable term for such a child, then I would expect that you'd be able to call a child an atheist, too.