All the messengers taught the same basic message of oneness of Allah and submitting to His will. Thus all messengers were Muslims and proclaimed the message of Islam. Every person is accountable to his or her own deeds in front of Allah on the day of judgment. It is the faith in Allah and His messengers and sincere submission to the commands of Allah that will lead the individual to attain the mercy and grace of Allah and will take him to paradise. Thus Islam rejects the notion of original sin and sacrificial atonement.
Every newborn is pure and free from sins.
When an individual attains the age of puberty is when his or her accountability starts. The message of Islam is universal. It is the same message proclaimed by all prophets of Allah. Unlike all other prophets, Muhammed has been sent to the entire humanity. The message of Islam is for all people.