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Zero-age main-sequence (ZAMS) stars are considered to have enormous starspotsand show strong chromospheric emission lines because of their strong surfacemagnetic field. We discuss the dynamo activities of ZAMS stars with respect totheir periodic light variation caused by a starspot and with respect to thestrength of the chromospheric emission lines. The light curves of $33$ ZAMSstars in IC 2391 and IC 2602 were obtained from \textit{TESS} photometric data.The light curves can be grouped into the following four categories: singlefrequency, possible shape changer, beater, and complex variability. Theamplitudes of the light curves are $0.001-0.145\,\mathrm{mag}$, similar tothose of ZAMS stars in Pleiades. The starspot coverages are $0.1-21\%$. Wefound that the light variations and Ca\,\emissiontype{II} emission linestrength of ZAMS stars in IC 2391, IC 2602, and the Pleiades cluster are aslarge as those of the most active superflare stars and two orders larger thanthose 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 tothose of the ZAMS stars of ages between $30$ and $120\,\mathrm{Myr}$. ZAMSstars with a single frequency or possible shape change in the light curve tendto have both large light variation, indicating large spot coverage, andsaturated Ca\,\emissiontype{II} emission line strength. ZAMS stars with beat orcomplex variability have small spot coverage and a faint Ca\,\emissiontype{II}emission line. We also detected $21$ flares in the \textit{TESS} light curvesof $12$ ZAMS stars in IC 2391 and IC 2602, where most of these stars havesaturated chromospheric Ca\,\emissiontype{II} emission lines. The energies ofthe flares are estimated to be $\sim 10^{33}-10^{35}\,\mathrm{erg}$, which iscomparable with the energy of a superflare.

Starspots, chromospheric emission lines, and flares of zero-age main-sequence stars