[@FalkiThomas] How old is Zack? In normal circumstances they are two years apart.

Zack is 16-23 in Crisis Core & Before Crisis
Cloud is 15-21 in Crisis Core & Before Crisis
Cloud is 21 in Final Fantasy VII
Cloud is 23 in Advent Children
Cloud is 24 in Dirge of Cerberus

Cloud is a minimum 24 years old in the RPG, maybe older.