Short answers, pulled from the story.
A regent is a person appointed to exercise the powers of a monarch temporarily, acting as a substitute ruler. Regencies arise when a monarch is a minor, ill, absent from the country, or when the throne is vacant or the identity of the legitimate monarch is disputed.
Prem Tinsulanonda became the oldest regent of any nation in 2016, at the age of 96. He served as regent for Rama X of Thailand, who chose not to formally ascend the throne until the mourning period for his father had ended. The previous record was held by Prince Regent Luitpold of Bavaria, who was 91 at the end of his regency.
The British Regency era takes its name from the period from 1811 to 1820, when the future George IV governed as Prince Regent while his father George III was incapacitated by mental illness. The label Regency era, and associated terms like Regency architecture, are commonly applied to a somewhat wider period than the strict years of the regency.
Liechtenstein is described as the only country with an active regency, operating under Alois, Hereditary Prince of Liechtenstein.
In the Society of Jesus, a regent is a Jesuit in formation who has completed his novitiate and philosophy studies but has not yet begun theology studies. Regents are typically assigned to teach in schools or other academic institutions during this stage.
The Board of Regents of the University of the State of New York administers all activities related to public and private education from pre-kindergarten through postsecondary level, as well as professional licensure, across New York State. Its appointed members each hold the title regent.