  Newly-formed planetary systems with ages of <10 Myr offer many unique
insights into the formation, evolution, and fundamental properties of
extrasolar planets. These planets have fallen beyond the limits of past
surveys, but as we enter the next decade, we stand on the threshold of several
crucial advances in instrumentation and observing techniques that will finally
unveil this critical population. In this white paper, we consider several
classes of planets (inner gas giants, outer gas giants, and ultrawide
planetary-mass companions) and summarize the motivation for their study, the
the observational tests that will distinguish between competing theoretical
models, and the infrastructure investments and policy choices that will best
enable future discovery. We propose that there are two fundamental questions
that must be addressed: 1) Do planets form via core accretion, gravitational
instability, or a combination of the two methods? 2) What do the atmospheres an
interiors of young planets look like, and does the mass-luminosity relation of
young planets more closely resemble the "hot start" or "cold start" models? To
address these questions, we recommend investment in high-resolution NIR
spectrographs (existing and new), support for innovative new techniques and
pathfinder surveys for directly-imaged young exoplanets, and continued
investment in visible-light adaptive optics to allow full characterization of
wide "planetary-mass" companions for calibrating planet evolutionary models. In
summary, testing newly proposed planet formation and evolutionary predictions
will require the identification of a large population of young (<10 Myr)
planets whose orbital, atmospheric, and structural properties can be studied.
