  We present results from new deep HST/ACS photometry of I Zw 18, the most
metal-poor blue compact dwarf galaxy in the nearby universe. It has been
previously argued that this is a very young system that started forming stars
only < 500 Myr ago, but other work has hinted that older (> 1 Gyr) red giant
branch (RGB) stars may exist in this galaxy. Our deeper data indeed reveal
evidence for an RGB. Underlying old (> 1 Gyr) populations are therefore present
in even the most metal-poor systems, implying that star formation started at z
> 0.1. The RGB tip (TRGB) magnitude and the properties of Cepheid variables
identified from our program indicate that I Zw 18 is farther away (D = 19.0 +/-
1.8 Mpc) than previously believed.
