I confirmed with the official call centre by email. They replied me that:
Don’t worry if you have your ISIC card with current date, you can buy your tickets for Machupicchu there isn’t age limit.
If you have any question, write us.
Alright, as long as you are student and have a valid ISIC card, you can buy tickets at a half price. But you have to go to the official ticket centre located in Cuzco. You can’t buy it online. The address of the office is bellow:
Ministerio de Cultura Cusco
Address: Av. De la Cultura Nº238 Condominio Huascar-Wanchaq
Mon – Fri 07:00 – 18:30
Sat 07:00 – 12:00 (except holidays)