Question:
Genesis 8:21 The LORD smelled the soothing aroma; and the LORD said to Himself, "I will never again curse the ground on account of man, for the intent of man's heart is evil from his youth; and I will never again destroy every living thing, as I have done.

It Seems to me that the implication here is that a man's heart is not evil before his youth, such as infancy.

Answer:
Are you certain that the Hebrew word used here means "teenager" or "child older than 4"?
"Youth" could mean "beginning," but I know nothing about Hebrew, so I will stop here.
EDIT: even if we stick with the English, Dictionary.com gives me this definition of youth:
5. the first or early period of anything
Answer:
Your interpretation is really playing semantics here. "Youth" doesn't literally mean "child", as MrCrabby mentioned already. That's too much of a literal interpretation. Just like interpreting "yom" as "day", literally. Anyway, there's still no implication there. It's not logical to say that just because something was some way after point X, that it couldn't be that way before point X.
Anyhow, that's still taking the bit of verse out of context. Look at it within the rest of the verse. Does that make any sense? Here's what it would boil down to:
"God saw Noah's sacrifice. He said, 'Okay, I won't destroy man, because man always intends to do evil.' "
Hold on...does this mean that God's acting like, "Okay, so he's going to always do evil anyway, so why bother?" Of course not. So that interpretation's not correct anyway.
As for the correct interpretation? I'm not sure. That's a very confusing verse...
Answer:
Well, I don't see this Hebrew word used for infancy anywhere else in the Bible. But I'm tired and sick so I may be missing some stuff.
The Hebrew is somewhat vague, na`uwr does mean youth or early life. The fact though, is that there are words that are commonly used to describe infants. Such as `uwl, which refers to "infants" or "sucking child". I see no instance where the term presented would refer to infant too. I think he might be on to something.
Answer:
Here's a thought on this subject. If any of you have children, have you ever had to teach them how to misbehave? Or how to disobey? We, as parents, spend the vast majority of our time teaching our children how to be good and kind. We strive hard to counteract the bad in our children so that they will live good lives. My three year old son knows that I do not allow him to hit his younger brother, yet he still does, almost daily. I probably spend a good half hour each day disciplining him on this fact. I believe, yes, there is evil in man's heart from the beginning. I believe that in this verse, God is saying he is not going to keep destroying the human race simply because we aren't listening to what he is saying to us. He is giving us a chance at redemption, a chance for Jesus. I would continue, but it's early and I have to finish getting ready for work.