The planet Barrayar had a Regent elected by the Joint Council in case the ruling Emperor was underaged (i.e. a "mere boy"). A Regent's charge was ended when the Emperor reached his majority at the age of twenty.
Aral Vorkosigan served as Regent during Gregor Vorbarra's youth. There had not been a Regent for 120 years by that point; there was some speculation as to what old protocols would be dusted off.