Hello Kein,
One suggestion I have is for you should spend some time researching your career interests. Cosmology is a branch of physics that studies the macroscopic evolution of the universe. It deals with ideas of inflation, the conditions of the early universe, etc. It is an academic discipline, not a profession and as such does not require certification (at least in the way that say, a medical doctor requires certification by a national body in order to practice). Aerospace engineering is a branch of engineering having to do with flight. Astronomy is a science that is based on observations of the universe. While these areas can be somewhat related, they are very different, so it is worth doing some reading and taking some fundamental physics courses to figure out which areas really capture your interest.
One thing about astronomy is that you don't need to be a professional to make an active contribution to the field. Many amateurs have made important discoveries.
With respect to age, 30 is nothing to worry about - even if you have to start from scratch.
I'm not sure what your last question is asking with respect to a mature student's thesis. When you go to graduate school, a thesis results from unique research that you have done to advance the knowledge in a particular field. While it is often an extension of previous work, there is almost always a new and unique element to it.