1) this is an interesting question.
If you really think about it, the incest law is not enforced, it can't be. When two people want to get married, they dont have to go through DNA testing to do so, they're trusted that they are. But some people could be marrying relatives out that and have no idea that they are. Thats why it is a social taboo. Everyone knows from a young age that you don't do those things to family member, because its wrong. In fact, the incest taboo is universal, among all cultures, because it has detrimental effects for children. Its not about biological attraction, its about the results for the children.
Homosexuality is completely victimless, no one is hurt by homosexual attraction itself. It is not unnatural either, because you will notice that homosexuality occurs in nature. Many animals have been known to practice homosexuality.
As for older women who have children, that is certainly something that cannot and should not be controlled. If you are a theist, you probably believe that God has a plan and that when God gives a woman a baby it is for a reason, so I dont see the issue here. From an atheist perspective, I certainly think that women should TRY to have babies when they are younger, because its safer for her and for her child, but it should not be enforced. What would you suggest doing, forcing all women over 40 to have hysterectomies? Its impossible.
In other words, incest laws are really useless. Children in ALL cultures around the world are taught from a young age that incest is wrong. Its a universal social taboo, and a law is not needed. Why is it taboo? For about a thousand evolutionary reasons that I can't really explain, you might want to ask this in the biology section because honestly they can give you a great answer.
2) gambling - shouldn't really be an age limit, children dont have a lot of money anyway, there really is no reason for an age limit. You should be able to do with your money what you want.
voting - definitely should be an age limit because most children under the age of 18ish are not interested and know nothing about politics. I know that when I first voted, I was pretty much the only one among my friends who knew what the party platforms were (or cared to know) and I we were all 18. I dont think that law really hurts anyone or takes away any real rights.
drinking - absolutely not, there should not be a drinking age. I dont wanna go off on a tangent about this because I definitely could, but I think that drinking ages CREATE the problem, they dont help at all. I know from my experience as a teenages, drinking was only "cool" BECAUSE it was illegal. It was dangerous and new and exciting, so everyone loved it. I turned 19 last month (legal drinking age in Ontario) and I can tell you that my drinking has actually gone way down. Why? because its not dangerous anymore. I know its silly, but its true.
one of my best friends grew up in a home where her mother had let her and her sibblings drink wine at dinner since they were young, and she is the most responsible drinker I know. Its because it wasn't a novelty to her when her friends started drinking. She knew how to use alcohol correctly, and she is very responsible. I think if all parents were like this, and taught their children how to consume alcohol responsibly, underage drinking would by a non-issue.
sex - Where I live, you can legally consent to sex at 14 years old. I think that this is a good age, because most people don't want to have sex before that age anyway. There should be laws against older men having sex with younger girls (and vice versa, sorry) as there currently are because it prevents children from being taken advantage of. But I dont think that the age for consensual sex should be any older than 14 because at 14 people should be able to decide to have sex.
*Note: all of the above is my opinion, and MY opinion only! Please dont apply this to all atheists, because I am only speaking for myself.
Sorry this was so long!