  Zero-age main-sequence (ZAMS) stars are considered to have enormous starspots
and show strong chromospheric emission lines because of their strong surface
magnetic field. We discuss the dynamo activities of ZAMS stars with respect to
their periodic light variation caused by a starspot and with respect to the
strength of the chromospheric emission lines. The light curves of $33$ ZAMS
stars in IC 2391 and IC 2602 were obtained from \textit{TESS} photometric data.
The light curves can be grouped into the following four categories: single
frequency, possible shape changer, beater, and complex variability. The
amplitudes of the light curves are $0.001-0.145\,\mathrm{mag}$, similar to
those of ZAMS stars in Pleiades. The starspot coverages are $0.1-21\%$. We
found that the light variations and Ca\,\emissiontype{II} emission line
strength of ZAMS stars in IC 2391, IC 2602, and the Pleiades cluster are as
large as those of the most active superflare stars and two orders larger than
those of the Sun, and are located on the extensions of the superflare stars.
These results suggest that superflare stars link the properties of the Sun to
those of the ZAMS stars of ages between $30$ and $120\,\mathrm{Myr}$. ZAMS
stars with a single frequency or possible shape change in the light curve tend
to have both large light variation, indicating large spot coverage, and
saturated Ca\,\emissiontype{II} emission line strength. ZAMS stars with beat or
complex variability have small spot coverage and a faint Ca\,\emissiontype{II}
emission line. We also detected $21$ flares in the \textit{TESS} light curves
of $12$ ZAMS stars in IC 2391 and IC 2602, where most of these stars have
saturated chromospheric Ca\,\emissiontype{II} emission lines. The energies of
the flares are estimated to be $\sim 10^{33}-10^{35}\,\mathrm{erg}$, which is
comparable with the energy of a superflare.
