I see this as a common first world problem: Adults lying to children. I have experienced this so much. It might have happened to you. Share your experiences about these kind of situations. I have a few examples:

When I was in first grade, somebody told me "Alcatraz" was related to math. I eventually found out it was a famous prison that has a lot of controversy behind it.

Big surprise, in second grade, I asked where babies come from. My mother told me that I was born from her marrying my father. I thought, " Having a wedding gets women pregnant? WOW" 

Well, my parents have always lied about their age until I was like 9 or so. One day my mom was like 42 years old and the next day she was 50.

Yeah my parents would do that type of stuff just to keep me away from doing something even though nothing bad would happen as a result. Most parents tell little lies like that just to keep their kids away from certain things, and they soon find out that as they grow older and pick up on it and learn new things, it isn't as easy to do.

Its funny because my brother is 7 and I get to watch my mom say some of the same things to him, soon enough he will figure it out though. 

Many times that has happened to me.

Parents often lie to children for several reasons such as:
- forcing them to do what they tell them to do secretly
- children can be often gullible in general
- children may know the truth behind lies but they can't really explain it (their brain is still developing in the process)
- they don't want their children to be influenced on certain things that they won't like
- to hide something
At least those are some of the reasons at the top of my head...

I remember a time when my dad would tell me that milk will help me grow taller if I keep drinking it but I never grew taller...  

Are they lying or are they delaying the truth?  I think there is a big difference.

Explaining to a 2nd grader the process of impregnation and birth is a challenge.  Imagine the emotional, cultural, physiological and sexual conversations you're going to have with a 7 year old who probably cannot handle the abstract theory of zygotes, embryos, and the whole process.

The Easter Bunny and Santa Claus?  I don't know if that's lying either.

I don't remember any adult lying to me directly without some reason or another.  Nothing was ever malicious.  It's an interesting question but I think I lived a life of honesty and fairness.

warmaker : It is still the same thing. Delaying the truth is a way of lying to stop someone from knowing something at a younger age. Sure, the intentions may be different, but truthfully, they are one and the same. I also have something against your second "paragraph". While people may think that a child would be unable to handle something such as that, there really would not be an issue. It might be tough to explain the whole science behind it all, but the actual action would take mere seconds to explain. Without having previously been molded by society, learning about sex in general would be seen as just another thing to learn. People attach a sort of negative connotation to it (along with good connotations, but that is not what I am going for here), despite the fact that sex is necessary to sustain life, but without children having been influenced by the negative connotation, they would never see it as an issue, and would probably not even find it interesting. Delaying that truth is about as unnecessary as you can possibly get. 

Going back to the whole thread, I have been lied to a few times. I will consider the Easter Bunny and Santa Clause as lies, as they definitely told me that they existed. When I found out the truth, I was only disappointed, but the lies did make me feel better for a few of my younger years. I was probably lied to about the whole baby-making process, especially after seeing Dumbo, but I cannot really remember. I might have actually been told at a really young age, and not cared too much about it. 

When I was a kid, my Dad lied to me a lot, but the biggest lie was when he told me that my Mom never wanted to have me and that she would leave and never come back for me. That's the only time I can ever think of.

I don't remember being lied to, but I do lie to my niece.
I usually tell her if she misbehaves that she will turn into a boy.
I tell her that my little brother used to be my little sister and he misbehaved too much.