Because so far (as far as I know) we haven't found any sings of life from these very old star systems and because I went with the assumption that prior to the start of life on earth, the conditions in the universe would have been to bad for life to start development anywhere else. Also I formed my theory with no professional research or knowledge about the functioning of the universe and the formation of life and just based my theory on the odd YouTube video on the matter and my school education.

I have a different theory: considering how far away the closest planet with life probably is and assuming life on that planet started at a similar time as life on earth ( not from that perspective of earth but from the universe at large ), could we actually already observe the relevant changes to the planet that we could observe from our distance or is life just not yet observable due to the distances and times involved in observing other interstellar planets?