My parents, a’h, are no longer alive and sometimes I attend a minyan, of mostly young people, and no one says kadish. Am I obligated to do so? If so, do I say every Kadish. If I am not obligated, should I anyway (it’s a good thing to do)?
You are not obligated to say kaddish, but you should say the kaddish after aleinu. This is because there is a need for a kaddish to be said after saying pesukim, and there are pesukim in aleinu. Technically even a person that has parents can say it, however this would need the consent of the parent to do so. Therefore, it should be said by a person that has a parent that already died. This would apply to all three tefillos.
Rema O:CH 132-2. Note there are other opinions regarding saying the kaddish after the Shir Shel Yom, and Pitum Haketoras, see Pischei Teshuvos 132-16.