We present a photometric study of stellar populations in the blue compact dwarf (BCD) galaxy I Zw 18 from the Hubble Space Telescope observations in 2003. The resulting color-magnitude diagrams (CMD), obtained with the DOLPHOT package, reach ≈ 29 mag in V and I. No dominant population of red giant branch (RGB) stars was revealed in the halo of I Zw 18, implying that all stars in I Zw 18 are quite young and have not scattered yet in the large volume. Thus I Zw 18 appears to be the only galaxy, where the halo of old stars is not detected. The derived stellar luminosity functions show that the tip of the possible red giant branch (TRGB) stars has a different location on the CMDs for the main body of I Zw 18 (I_TRGB = 27.2 mag) and the C component (I_TRGB = 26.5 mag). Using the theoretical dependences of M_I,TRGB on the age and color of the TRGB, we resolve this contradiction, proposing the hypothesis that the formation of RGB stars in I Zw 18 is recently started and red giants have different ages in the main body and the C component. We found the distance modulus for I Zw 18 to be (m - M) = 30.2—30.7 mag, corresponding to the distance D ≈ 11 — 14 Mpc. We estimate the age of oldest stars in the main body to be ~1.2 Gyr, while in the C component their age ranges from 1.5 Gyr to 2 Gyr. Besides, we derived the CMDs of the BCD galaxies NGC 2366, NGC 4068, UGC 6456 and SBS 1415+437, as well as found the luminosities of their brightest blue (BL) and red (RSG) supergiants, and asymptotic red giants (AGB), which allows us to perform an independent check of the distance to I Zw 18 derived from the TRGB. It is found that the distance to I Zw 18 derived from the brightness of BL stars and RSG stars is 11—12.6 Mpc and 12.6—14.5 Mpc, respectively. On the other hand, the distance determined from the brightness of AGB stars is more uncertain and larger, D = 13.2—16.6 Mpc.