[sircastor]

Heck, I didn’t figure out what career I was interested in until I was nearly 30. I didn’t complete my bachelor’s degree until I was 40.

28 is definitely not too old, and if you change your mind at 35, you won’t be too old then either.

I would encourage you though to figure out if you really need to pursue your interest in biology or mathematics professionally? Depending on what your interest is here, you may be able to do these as a hobby, which frees you from doing the research someone else wants you to do, rather than what you’re interested in. Your career doesn’t have to be your passion.