Capital punishment in the United States:

Sixteen was held to be the minimum permissible age in the 1988 Supreme Court decision of Thompson v. Oklahoma. The Court, considering the case Roper v. Simmons in March 2005, found the execution of juvenile offenders unconstitutional by a 5–4 margin, effectively raising the minimum permissible age to 18. State laws have not been updated to conform with this decision. In the American legal system, unconstitutional laws do not need to be repealed; instead, they are held to be unenforceable. (See also List of juvenile offenders executed in the United States)

Please answer a question about this article. If the question is unanswerable, say "unanswerable". According to Thompson v. Oklahoma, what was the oldest age at which a person might be executed?
unanswerable