Excellent question!
Special thanks to our SME Doug for taking the time find this information.
AR 600-8-22, Awards, Para 1-14c
No military decoration, except the Purple Heart, will be awarded more than 3 years after the act or period of service to be honored. The Purple Heart differs from all other decorations in that an individual is not “recommended” for the decoration; rather he or she is entitled to it upon meeting specific criteria. Exceptions to the time limit include decorations approved under 10 USC 1130 and the provisions in sub paragraphs d and e below.
Para 2-8 Purple Heart: (This is the goto section to see if the Soldier meets the criteria for the award.
Subpara c: While clearly an individual decoration, the Purple Heart differs from all other decorations in that an individual is not “recommended” for the decoration; rather he or she is entitled to it upon meeting specific criteria.