The high-energy emission from low-mass stars is a product of stellar magnetism and turbulent convection as well as an input to the dynamics of planetary atmospheres. Observing this high-energy emission and understanding its formation are vital to understanding the physics of both stars and planets. One limitation to our knowledge of planetary atmospheric escape is the lack of observed extreme ultraviolet spectra for most stars, and to overcome this obstacle I present a method to use more accessible observations to infer the unobserved extreme ultraviolet. Then I measure the variability of emission from the upper layers of the stellar atmosphere for a number of stars across a range of rotation periods to characterize one aspect of how magnetic heating processes seem to collectively wane as stars age and spin down. That spin-down over time takes place after an initial phase of high activity during a star’s youth, an important period of time for early planetary evolution, and so I combine my method of extreme ultraviolet inference with existing studies of solar-type stars to estimate the cumulative irradiation of planets orbiting the young Solar analog V1298 Tau. Finally, I extend this approach of using stars across time and rotation period to map out how the high-energy spectrum evolves for M dwarfs, the most common type of star and exoplanet host.