In which scenario is imprisonment mandatory for a capital felony?

Imprisonment is indeed mandatory for a capital felony under specific circumstances defined by law. In this case, when the individual is less than 18 years old, it represents a critical age factor as many legal systems prohibit the execution of minors or the imposition of certain harsh penalties, including long-term imprisonment, for those under this age.

This aligns with principles of juvenile justice, which strive to treat young offenders differently, recognizing that their psychological and moral capacities differ from adults. Therefore, if a capital felony is committed by an individual under the age of 18, legal statutes often stipulate that the punishment is not the death penalty or life imprisonment without parole, but rather alternatives that are deemed more appropriate for juveniles.

In contrast, other scenarios listed, such as the offense occurring during a violent crime, the individual's age being 21 or older, or when the offense was a property crime, do not inherently invoke mandatory imprisonment for a capital felony according to typical legal standards. Each of these scenarios would require additional context and factors to determine the appropriate legal consequences.