  Measuring the evolution of X-ray emission from pre-main sequence (PMS) stars
gives insight into two issues: the response of magnetic dynamo processes to
changes in interior structure and the effects of high-energy radiation on
protoplanetary disks and primordial planetary atmospheres. We present a sample
of 6,003 stars with ages 7-25Myr in ten nearby open clusters from Chandra X-ray
and Gaia-EDR3 surveys. Combined with previous results in large samples of
younger (<5Myr) stars in MYStIX and SFiNCs star forming regions,
mass-stratified activity-age relations are derived for early phases of stellar
evolution. X-ray luminosity (Lx) is constant during the first few Myr, possibly
due to the presence of extended X-ray coronas insensitive to temporal changes
in stellar size. Lx then decays during the 7-25Myr period, more rapidly as
stellar mass increases. This decay is interpreted as decreasing efficiency of
the alpha^2 dynamo as radiative cores grow and a solar-type alpha-Omega dynamo
emerges. For more massive 3.5-7Mo fully radiative stars, the X-ray emission
plummets indicating lack of an effective magnetic dynamo. The findings provide
improved measurements of high energy radiation effects on circumstellar
material, first the protoplanetary disk and then the atmospheres of young
planets. The observed X-ray luminosities can be so high that an inner
Earth-mass rocky, unmagnetized planet around a solar-mass PMS star might lose
its primary and secondary atmospheres within a few-several million years. PMS
X-ray emission may thus have a significant impact on evolution of early
planetary atmospheres and the conditions promoting the rise of habitability.
