Concessionary Season Ticket Holders

I like many attend matches with my son (who pays for an U16 season ticket) and my father (who pays for an over 65 ticket). However on odd occasions one or the other doesn't go and therefore in previous seasons I have taken an adult friend insted who would not have gone otherwise.

This season I notice that the concessionary is printed on the actual match ticket part of the stub. Does this mean an adult would not be allowed to use this, and if this is the case is there a way that this question can pay the uplift in price rather than just missing out all together?

I would advise checking this with the ticket office.