I turned 25 years old the day before yesterday. Can I still purchase a Youth travelcard?

No, as you have already reached your 26th year. On the other hand, you may use your ‘Youth’ monthly or annual travelcard up to its date of expiry, provided the first day of validity of the existing travelcard is before your 25th birthday. Multiple-journey passes, multiple day passes and group passes at a reduced price can be used up to the day before your 25th birthday. We recommend that young people with reduced tickets carry an official ID that states their age.