Are you referring to the GO tickets 1 / 10 for under 26 year olds? These are valid only for SNCB trains.
www.b-rail.be/nat/F/tarifs/gopassformulas/index.php
There are 3 ways of getting from Brussels (Zaventem) airport into the city, but from what you describe, you are best sticking to the buses of STIB, as they operate all trams and metro lines in Brussels, as well as most bus lines or the train then metro. The other alternatives are the De Lijn buses.
Here are a few ticket prices, to help you.
Single ticket for STIB Airport express bus 12/21 costs 3.50 if purchased from a GO ticket machine (not to be confused with GO SNCB ticket for under 26s). It can be used to continue by metro/tram/bus in Brussels, so long as you start that journey less than 60 minutes after vaildating the ticket on the 12/21 bus.
The same Single ticket for STIB Airport express bus 12/21 costs 6.00 if purchased from the driver of the 12/21 bus.
A single NON STIB airport express ticket, called a JUMP 1, costs 2.00 if bought in advance from a GO machine or a KIOSK/BOOTIK ticket office in a metro station. The same ticket JUMP 1 costs 2.50 from a bus/tram driver. You cannot enter most metro stations without buying a ticket, so you have to go the GO machine there or KIOSK/BOOTIK.
a 5 trip ticket for STIB, called a JUMP 5, costs 7.50, remember not valid for the airport 12/21 bus.
A 10 trip ticket for STIB, called a JUMP 10, costs 13.00, remember not valid for the airport 12/21 bus.
A 1 day STIB ticket, not including airport bus, costs 6 euro. Not sure if it is 24 hours, I suspect you have to buy 2 of these tickets if you go over into 2 days. So only worth it if doing more than 3 journeys in a day, not using the 12/21 airport bus.
Here is the information on STIB tickets
www.stib.be/tickets.html?l=en
A single SNCB ticket for the train from the airport to Brussels costs 5.40. A return is 10.80.
There is apparently a GO machine to buy tickets for the 12/21 bus somewhere in the airport. I do not know where, assuming near the bus stop. Otherwise you will have to find 6.00 to pay the driver.