It is known that adult and sub-adult Swainson's Hawks show a broad sub-terminal tail band compared to 1st-year birds who show a narrow band at the tip of the tail; this is often a good way to tell 1st-year birds apart in trickier photos or field situations. However, some 1st-year birds have a wide sub-terminal tail band similar to older ages, so beware of this and try to include other traits as well when trying to tell the age of a Swainson's Hawk to confirm your suspicians. Check out the images in the gallery and note the light-morph 1st-year bird with the wide band near the tail tip, and the typical narrow tail band on the darker bird. Also compare it to the adult shown that has the dark bib on the upper chest and a wide sub-termminal tail band.
One more caution, it has been said that adult Swainson's Hawks can be sexed by the width of their sub-terminal tail band, but that is not quite so accurate since there is overlap between the sexes, and it doesn't hold true particularly for the darker morphs. I wish it did, but like everything regarding raptors - almost nothing is 100% reliable and the natural variation and overlap is often great!