  Flavor Democracy Hypothesis favors existence of iso-singlet quarks and
vector-like charged leptons. Their observation at future colliders could shed
light on the nature of mass and mixing patterns of known leptons and quarks, as
well as Higgs boson itself. Vector-like quarks are extensively searched by
ATLAS and CMS collaborations. Unfortunately, this is not the case for
vector-like leptons, while they have actually similar status from phenomenology
viewpoint. We argue that vector-like charged leptons should be included into
the new physics search programs of future energy frontier colliders. It is
shown that pair production at the HE-LHC with decay of one of the leptons to Ze
and another to He channel, followed by H \rightarrow bb and Z \rightarrow
\mu\mu decays will give opportunity to scan masses of iso-singlet and
iso-doublet charged leptons up to 0.9 TeV and 1.5 TeV, respectively. FCC will
extend this region up to 2 TeV for iso-singlet and 3.6 TeV for iso-doublet
charged leptons.
