Hello Phaikouthouth61
To determine whether you are eligible to purchase this offer, your actual age on the day of the first validity is decisive and considered. Provided you purchase the Half Fare Card for Youth before your 25th birthday, your Half Fare Card remains valid for the entire year.
However, please keep in mind that, when purchasing a Half Fare Card, you sign up for an auto-renewal subscription. Make sure, you are cancelling the auto-renewal on time yet at least 1 month before the expiration date of your Half Fare Card; otherwise the contract will continue, and you are subject to pay for the adult Half Fare Card for another year.
https://www.sbb.ch/en/tickets-offers/travelcards/half-fare-travelcard.html
Does that make sense?
Kind regards,
Roland