Boy Who Killed Gets 7 Years; Judge Says Law Is Too Harsh
State judge rules that youngest American ever convicted of murder as an adult will serve seven-year sentence at maximum-security juvenile detention center and then be freed; Judge Eugene Arthur Moore of Pontiac, Mich, rejects prosecutors' request that boy, Nathaniel Abraham, who was convicted of killing when he was 11, be sentenced to serve additional time in adult prison unless he has clearly been rehabilitated by age 21; judge says juvenile laws have become weighted too heavily toward punishment instead of rehabilitation; he calls for changes in 3-year-old state law that makes it possible for children of any age to be tried as adults in Michigan; at least 44 states have adopted laws since 1992 that allow more children to be tried as adults; photo (M)