Trying to imitiate English constructions in German doesn't always work :-)
to think about s.th. = über etwas nachdenken
to think (that) = denken, dass
it doesn't matter = es ist egal, es ist gleich
So: "Ich habe oft gedacht, dass es zehn Minuten, nachdem man gestorben ist, egal ist, wie lange man gelebt hat." Yes, that's a difficult sentence structure. Mark Twain would have loved it... So one would probably express it differently, e.g. "Ich habe oft gedacht, dass es völlig egal ist, wie lange man gelebt hat, sobald man tot ist."
Apart from that, the translation is mostly ok. You should drop the "also" in the last sentence. That's ok in colloquial speech, but verboten in writing :-)