It seems that a reduced price for transportation tickets within Stockholm is available for young people up to the age of 20 years. But I can't find out whether it is up to 20 (including 20) or if you have to be younger than that (so up to and including 19, but not including those aged twenty).
I am 20 and I want to buy a travel card for 72 hours but I don't know whether or not I can buy the ticket for reduced price.