Teenagers in their free time will often participate in activities such as playing sport with a team, practice music or take theater lessons.
Scouts or guides are a good choice to meet other teenagers and spend time outside home.
Here is a list of all the different sport clubs around Brussel: http://www.inforjeunesbruxelles.be/liste-des-clubs-sportifs-classes-par-commune-79
Here are the way to find a scout or patro unity close to your house: http://www.patro.be/nos-services/googlemap/
https://lesscouts.be/parents/trouver-une-unite.html
Useful websites for other cultural activities in Brussel:
http://www.pianosharmonieatelier.be/liens-utiles/
https://www.quefaire.be/stages,cours/0_1_2_1_439.html
Other general information for teenagers in Brussel:
http://be.brussels/culture-tourisme-loisirs/bruxelles-pour-les-jeunes