I think most of the school mention DAT not older than two years. This was my plan originally (taking DAT my sophomore-junior year), but my pre-health advisor told me that if I sent an old DAT they will think I am applying again!!! But now that I went through the whole process I realize that it is not true, when you fill out your application and secondary applications they ask you if this is the first time applying or not, so old DAT just tell them you took the DAT early not that you are applying for the second time. I will say if you have your prerequisites with good grades (A) and you feel confident you know the material, go ahead study during the summer of your sophomore year and take the test, if you don't do good you have pleny of time to study again, and now you have experience.
On the other hand I can tell you I took the DAT in my senior year, and at that time I had a lot of high level science classes, which were very useful in the biology section, so if you wait a little longer you have the benefit of getting more knowledge and some subjects will become more clear. But I think that if you get good sources to study like APbio cliffs or Shaums biology, you will be fine. Study, study, study and you will fine. GL