  The blue compact dwarf galaxy I Zw 18 is one of the most metal-poor ($Z \sim
3% Z_{\sun}$) star-forming galaxies in the local Universe. Its evolutionary
status has sparked debate within the astronomical community. We aim to
investigate the stellar populations of I Zw 18 in the near-IR using
JWST/NIRCam's high spatial resolution and sensitivity. Additionally, we aim to
derive the galaxy's spatially resolved star formation history (SFH) over the
last 1 Gyr and provide constraints for older epochs. We used DOLPHOT to measure
positions and fluxes of point sources in the F115W and F200W filters' images of
I Zw 18. To derive I Zw 18's SFH, we applied the color-magnitude diagram (CMD)
fitting technique SFERA 2.0, using two independent sets of stellar models. Our
analysis reveals three main stellar populations: one younger than $\sim30$ Myr,
mainly in the northwest star-forming (SF) region; an intermediate-age
population ($\sim 100 - 800$ Myr) in the southeast SF region; and a red and
faint population linked to the underlying halo, older than 1 Gyr and possibly
as old as 13.8 Gyr. The main body of the galaxy shows a very low star formation
rate (SFR) of $\sim 10^{-4} M_{\odot} \text{yr}^{-1}$ between 1 and 13.8 Gyr
ago. In the last billion years, I Zw 18 shows increasing SF, with strong bursts
around $\sim10$ and $\sim100$ Myr ago. Component C mirrors the main body's
evolution but with lower SFRs. Our findings confirm that I Zw 18 contains stars
of all ages, indicating it is not a young galaxy but has an old stellar halo,
similar to other BCDs. The low SF activity over the past billion years supports
the "slow cooking" dwarf scenario, explaining its low metal content. Currently,
the galaxy is undergoing its strongest SF episode ($\sim 0.6 M_{\odot}
\text{yr}^{-1}$) mainly in the northwest region, likely due to a recent
gravitational interaction with Component C.
