Am I wrong for thinking about using a different snapper?

Ok so I’m studying at Vic Uni right now and I catch public transport to and from campus everyday of the week. Last Friday, I was cleaning my bag and while I was looking through it, I found my old child snapper card and it still has money on it. I’ve been using an adult card with a tertiary discount on it, but the child card would still be cheaper. Unfortunately, I’ve already turned 18 and I have some reservations about not paying the correct fee. Should I just use the child one or give it away?