Tahl was born in mid- 2369 to career diplomats on Maravel. His mother is an expert on Federation-Cardassian relations and helped to negotiate the peace treaty between the Union and the Federation. His father was a diplomat but he has been many, many other things during Tahl's lifetime. His early childhood is largely undocumented (or at least not in his Starfleet profile) but according to him it was very loving and happy. Tahl's parents were part of the Federation relief mission assigned to Cardassia Prime in the wake of the Dominion War and this is where Tahl spent his life from ages 7 to 18 and considers it to be more or less "home".
Starfleet was not an obvious choice for Tahl and neither was physics but when Tahl attended lectures at the Central University of Cardassia he found himself drawn to the higher problems and theories of physics. Previously considering himself not at all science or math oriented, Tahl pursued his interests from a more literary perspective and specialized in Astronomy. He was accepted to Starfleet Academy the next year and began a six year course in an integrated dual bachelors and masters degree program in Physics and the Philosophy of Science.