Type Ia supernovae (SNe Ia) occur in both old, passive galaxies and active, star-forming galaxies. This fact, coupled with the strong dependence of SN Ia rate on star formation rate, suggests that SNe Ia form from stars with a wide range of ages. Here we show that the rate of SN Ia explosions is about 1% of the stellar death rate, independent of star formation history. The dependence of SN Ia rate on star formation rate implies a delay time distribution proportional to t-0.5 +/- 0.2. The single-degenerate channel for SNe Ia can be made to match the observed SN Ia rate-SFR relation, but only if white dwarfs are converted to SNe Ia with a uniform efficiency of ~1%, independent of mass. Since low-mass progenitors are expected to have lower conversion efficiencies than high-mass progenitors, we conclude that some other progenitor scenario must be invoked to explain some, or perhaps all, SNe Ia.