In general, I don’t think it is premature to be thinking about a career, but it is premature to be committing to one.
In fact, with a degree that is as ubiquitous as Psych, I would argue that it is very important. (Speaking as a parent of a Psych Major).
Forensic Psychology is an “applied” psychology discipline. It builds upon one of the “basic” disciplines.
At the undergrad level, it is a good idea to get some exposure to the application area that you are interested in (as well as other application areas for your major), to figure out what you really like. This can be done through applied coursework, internships and/or research/independent study.
The ability to take an applied course or two that exposes you to the area of Law as well as Clinical Methods is a big plus in deciding if you really like the field and it also serves as a differentiator among the millions of Psych grads out there.
One of the top academics in the field of Forensic Psych is Saul Kassin.
He is at Williams, but unfortunately he is in the process of retiring. I am not sure what the succession plan is, but Williams could also be an option as they have a course in Psychology and the Law and research opportunities, but no applied coursework that I am aware of.
http://web.williams.edu/Psychology/Faculty/Kassin/biography/index.html
In terms of other academics in the field, Sam Sommers (a behavioral psychologist) at Tufts is one of Saul Kassin’s pupils. He has won a number of teaching awards as well as a early career excellence award from the American Psychology Law Society. He also performs research in the area of Psychology and the Law as well as teaching courses.
http://www.ase.tufts.edu/psychology/people/sommers/
http://www.ase.tufts.edu/psychology/sommerslab/researchPublications/psychologyLaw.htm