  The zero-age main sequence (ZAMS) is a critical phase for stellar angular
momentum evolution, as stars transition from contraction-dominated spin-up to
magnetic wind-dominated spin-down. We present the first robust observational
constraints on rotation for FGK stars at $\approx40$ Myr. We have analyzed TESS
light curves for 1410 members of five young open clusters with ages between
25-55 Myr: IC 2391, IC 2602, NGC 2451A, NGC 2547, and Collinder 135. In total,
we measure 868 rotation periods, including 96 new, high-quality periods for
stars around 1 ${M_{\odot}}$. This is an increase of ten times the existing
literature sample at the ZAMS. We then use the $\tau^2$ method to compare our
data to models for stellar angular momentum evolution. Although the ages
derived from these rotation models do not match isochronal ages, we show these
observations can clearly discriminate between different models for stellar wind
torques. Finally, $\tau^2$ fits indicate that magnetic braking and/or internal
angular momentum transport significantly impact rotational evolution even on
the pre-main sequence.
