  In a multi-wavelength survey of $13$ quasars at $5.8\lesssim z\lesssim6.5$,
that were pre-selected to be potentially young, we find five objects with
extremely small proximity zone sizes that may imply UV-luminous quasar
lifetimes of $\lesssim 100,000$ years. Proximity zones are regions of enhanced
transmitted flux in the vicinity of the quasars that are sensitive to the
quasars' lifetimes because the intergalactic gas has a finite response time to
their radiation. We combine sub-mm observations from the Atacama Large
Millimetre Array (ALMA) and the NOrthern Extended Millimeter Array (NOEMA), as
well as deep optical and near-infrared spectra from medium-resolution
spectrograph on the Very Large Telescope (VLT) and on the Keck telescopes, in
order to identify and characterize these new young quasars, which provide
valuable clues about the accretion behavior of supermassive black holes (SMBHs)
in the early universe, and pose challenges on current black hole formation
models to explain the rapid formation of billion solar mass black holes. We
measure the quasars' systemic redshifts, black hole masses, Eddington ratios,
emission line luminosities, and star formation rates of their host galaxies.
Combined with previous results we estimate the fraction of young objects within
the high-redshift quasar population at large to be $5\%\lesssim f_{\rm
young}\lesssim 10\%$. One of the young objects, PSO J158-14, shows a very
bright dust continuum flux ($F_{\rm cont}=3.46\pm 0.02\,\rm mJy$), indicating a
highly star-bursting host galaxy with a star formation rate of approximately
$1420\,M_{\odot}\,\rm yr^{-1}$.
