A majority of states have banned juvenile executions.
This ceased after the 2005 Supreme Court decision Roper v. Simmons, which found juvenile execution unconstitutional as "cruel and unusual punishment".
Seven states have carried out a total of 21 juvenile executions since 1976, with Texas responsible for 13 of those.
Professor Scott, opponent of juvenile executions, said science, too, lent evidence bolstering the case for abolishing them.
Texas alone has accounted for 13 of the 22 juvenile executions since the modern era of capital punishment began in 1976.
That 6-to-3 opinion indicated in a footnote that its analysis would not be extended to juvenile executions.
Both states allow juvenile executions.
There have been an increasing number of public executions and death sentences - juvenile executions in particular.
Last week, for instance, the day after the EU deplored the hanging of Mr Hejazi, a minor, yet another juvenile execution took place.
First was an assessment of whether American society had formed a consensus against juvenile executions.