What are the advantages/disadvantages of a late 20s student entering traditional astronomy programs?
It depends on many things. Since you are asking about a ‘program’ rather than asking about employment prospects, I will assume that you have already completed an undergraduate program in physics. If you haven’t completed an undergraduate program in physics, your chances of taking the next step and entering graduate school are pretty poor. Training in a traditional program in astronomy begins in graduate school. There are so many students applying for graduate school in astronomy these days that graduate schools can be very selective about who they admit. Sadly, you already have one strike against you in that you didn’t enter graduate school immediately after finishing your undergraduate program. If, furthermore, your undergraduate education was not in physics this would be a second strike against your admissions package. I am sorry to be so brutal and frank about this. I think many people are completely unaware just how difficult a program in astronomy is. If I were contemplating a car
