The Nazis occupied most of France in 1940 (and the last soldiers left in 1945, e.g. La Rochelle). The movie starts ("First Chapter") in 1941 (shown on screen) with Colonel Landa interviewing LaPadite. He specifically asks him for Shosanna's age, LaPadite estimates it as 18 or 19.
In Chapter Two the Basterds are introduced. Lieutenant Raine says to his men that everyone knows that the invasion starts soon but their group will get into France even earlier. We then see them active in France and Hitler's reaction. A map is shown behind him, it doesn't show any allied occupied territory in France but it isn't historical in other regards too; it's more of delusion and propaganda. In general it shows the state of German occupied territory maybe 1943.
Chapter Three (Shosanna meets Zoller in Paris et al.) is set in 1944 (shown on screen).
The last scenes of the movie are shown at the front line that Colonel Landa, his driver, Lt. Raine and Donny are driving to in a truck so it takes place after the invasion. The main diversion in the timeline is the killing of a lot of high ranked Nazis up to Hitler, the rest seems to be intact. The actions after the massacre are speculative but probably not realistic; generally an unconditional surrender was mandated by the allied after the conference of Casablance, local negotiations could probably have happened nevertheless. But after the timeline diversion it remains speculative of course.
So, if in the beginning of the film Shoshanna was 18 or 19, at the end (the theater destruction) she would be 21 or 22.