5:12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned: 5:13 (For until the law sin was in the world: but sin is not imputed when there is no law.
5:14 Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come.
5:15 But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many.
5:16 And not as it was by one that sinned, so is the gift: for the judgment was by one to condemnation, but the free gift is of many offences unto justification.
5:17 For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.) 5:18 Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.
5:19 For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.
(King James Bible, Romans)
From these verses, and others, the Bible teaches that by one man, Adam, sin and thereby death, was passed down to all men. This is why Christ was born of a woman, and not a woman and a man, or he would have inherited Adam's sin. Every person born into this world was born with a sinful nature, although children are all welcome into Heaven, as Jesus said the kindom of Heaven is made up of such, once they reach an age of accountability, where they hear and understand the gospel, they are no longer 'exempt'. The whole argument in Romans is that Adam sinned and passed this 'disease' on to all mankind. ALL have sinned the Bible says, there is none righteous, no not one. This is why Jesus had to die, to pay for our sins, and why it says those who believe have eternal life and those who don't are condemned already. We all need His blood to pay for our sins.