  This paper reports an environmental analysis of 41 uniformly-selected
stripped-envelope supernovae (SESNe) based on deep ultraviolet-optical images
acquired by the Hubble Space Telescope. Young stellar populations are detected
in most SN environments and their ages are derived with a hierarchical Bayesian
approach. The age distributions are indistinguishable between Type IIb and Type
Ib while that for Type Ic is systematically younger. This suggests that the
Type Ic SN progenitors are more massive while the Type IIb and Type Ib SNe have
very similar progenitor masses. Our result supports a hybrid envelope-stripping
mechanism, in which the hydrogen envelopes of the SESN progenitors are stripped
via a mass-insensitive process (e.g. binary interaction) while the helium
envelopes are stripped via a mass-sensitive process (e.g. stellar wind of the
post-binary interaction progenitor). We also provide progenitor constraints for
three Type Ibn SNe and two broad-lined Type Ic SNe. All these results
demonstrate the importance of the very diverse mass-loss processes in the
origins of SESNe.
