HE WASN'T a nice person. He wasn't even a decent kid. By the time he was 6 years old, Charles Rumbaugh, who was executed in Texas this week, had committed his first crime. His first armed robbery was accomplished when he was 12. He spent most of his childhood in a series of reform schools and mental institutions, and when he was 17 he murdered a jewelry store owner in the course of a robbery. He is the first person in 21 years to be executed in this country for a crime committed before he became 18. Thirty-two others -- all convicted of murder -- are on death row throughout the country.

There is very little in the records of these young criminals that evokes sympathy. Their crimes are often more egregious, in fact, than those of older killers. One raped and killed a 76-year-old nun; another beat to death a paralyzed man confined to a wheelchair. It stands to reason that judges and juries would be more hesitant to impose capital punishment on young offenders, so the penalty is meted out only in the worst cases. The question remains, though, whether even those who favor capital punishment want to use it in cases where the crime was committed before the offender was an adult.

Capital punishment is legal in 35 states. Only six of them prohibit its use as a penalty for acts by those under 18. In Indiana, anyone over 10 when he commits a capital crime can be executed. In many states without a minimum, age is a factor at sentencing, though it has not always been decisive. And in three states, the age of an offender is not even considered.

The movement to change these laws and prohibit the penalty for under-18 crimes is not led by do-gooders and bleeding hearts but by the tough- minded American Bar Association. The ABA has taken no position on capital punishment generally but has vigorously opposed its use for very youthful crimes through a public education program and a lobbying effort in state legislatures. It is not necessary to be generally soft-hearted on crime or to condone any of the terrible murders involved in these cases to argue for a rational policy toward imposing the extreme and final penalty for youthful crimes. We have, for most purposes, set 18 as the age of maturity in this country. It is right, not to say humane and compassionate, to take that into consideration before sending a man to his death.