Christ's death is more of a payment for sins, it doesn't make you sinless from birth, and the effects of his death are only applied to those people who accepted him and believed that he indeed did die for their sins. obviously people born today would still be sinners from birth cuz they haven't believed nor do they know about christ as children, but as they grow up and mature, if they choose to believe that christ died for their sins, then they would be cleansed from all their sins, past present and future. Hence they'd be able to get salvation. Which is why Christ died for our sins. (In John 3:16 "For God so loved the world that he gave his only begotten son that whosoever believes in him shall not perish but have eternal life") (emphasis on 'believes in him')