Question: In what year was the last monarch under which Robert Waterton was a trusted servant of the House of Lancaster born?

Context paragraphs:
Joan Champernowne: Joan Champernowne (died 1553) was a lady-in-waiting at the court of Henry VIII of England.  She became a close friend to his sixth wife, Catherine Parr and married Sir Anthony Denny, who at the end of Henry's reign was widely considered to be his most trusted servant.

Baptist May: Baptist May (1628–1698) was a Royal courtier during the reign of Charles II of England.  He is said to have been Charles's closest and most trusted servant, largely as a result of his knowledge that the king did not like to be approached on matters of business.

Gerald Comerford: Gerald Comerford (c.1558–1604; also called "Gerard" or "Garrett Comerford") was an Irish barrister, judge and statesman of the late sixteenth and early seventeenth centuries.  He sat in the House of Commons in the Irish Parliament of 1585–6, and briefly held office as Chief Justice of Munster and as Baron of the Court of Exchequer.  Profiting from a close family association with the Earl of Ormond, he rose rapidly to become a trusted servant of the English Crown, and would probably have become a dominant political figure in the south-east of Ireland had it not been for his early death.

Henry VI of England: Henry VI (6 December 1421 – 21 May 1471) was King of England from 1422 to 1461 and again from 1470 to 1471, and disputed King of France from 1422.  The only child of Henry V, he succeeded to the English throne at the age of nine months upon his father's death, and succeeded to the French throne on the death of his grandfather Charles VI shortly afterwards.  Henry inherited the long-running Hundred Years War (1337–1453), where Charles VII contested his claim to the French throne.  Henry married Charles's niece, Margaret of Anjou, partially in the hope of achieving peace in 1445, but the policy failed, leading to the murder of William de la Pole, one of Henry's key advisors.  The war recommenced, with France taking the upper hand; by 1453, Calais was Henry's only remaining territory on the continent.

Sermon of Fadak: Sermon of Fadak (Arabic: خطبه فدک‎ ‎ ) is Fatimah's speech in the Islamic Prophet Muhammad's Mosque.  Fatimah Zahra delivered this sermon in the presence of Umar, Abu Bakr and a group of Muhammad's Companions who had gathered in Muhammad's Mosque.  She entered the mosque with group of women and her trusted servant.

List of the last monarchs in the Americas: The Native American hereditary leaders during this time are not included.  Those that are listed as former monarchs of what is now the continental United States were heads of European seated monarchies (or Mexican monarchs, such as Agustín I of Mexico and Maximilian I of Mexico) and themselves never set foot on American soil.  Others were indigenous monarchs, such as Malietoa Tanumafili I, Tuʻi Manuʻa Elisala and Liliuokalani of Hawaii; The last monarch of each state and territory may not be the last colonial ruler (i.e. Louis XV of France is the last French monarch of Louisiana, but Napoleon Bonaparte, prior to becoming Emperor, was the last ruler of Louisiana).  Also their end of reign may not be how the US acquired these states.  One state can have more than one last monarch, since each state may have been the product of many different acquisitions by the United States.  Different claims of a foreign country are taken into account.

Joanna Denny: Joanna Denny (died 2006) was a historian and author specialising in the court of Henry VIII of England.  Her books include "Katherine Howard: A Tudor Conspiracy" and "Anne Boleyn."  Her books are usually considered to be sympathetic towards these women.  She was published by Portrait Books, an imprint of Piatkus.  She is a descendant of Sir Anthony Denny, Henry VIII's trusted servant.  She died in 2006, shortly before the publication of her book on Anne Boleyn.

Hugh Waterton: Sir Hugh Waterton, (born circa 1340 [before 1373] – died 2 July 1409) was a trusted servant of the House of Lancaster.

Robert Waterton: Robert Waterton, (c. 1360 – 17 January 1425) was a trusted servant of the House of Lancaster under three monarchs, Henry IV, Henry V, and Henry VI.  As Constable of Pontefract Castle, he had custody of Richard II after that king was deposed.

Lucius Petronius Taurus Volusianus: Volusianus was a Roman citizen, apparently of equestrian origins, whose career in the Imperial Service in the mid-Third Century AD carried him from a relatively modest station in life to the highest public offices and senatorial status in a very few years.  He may have secured his first appointments before the Licinian Dynasty - (Valerian and his son Gallienus) - acceded to the Empire in 253 AD, but it was in the course of their reign that his upward progress achieved an almost unprecedented momentum and the second factor seems to have been a consequence of the first.  The nature of his relationship to the Licinii is uncertain, but it seems likely that a common origin in the Etruscan region of central Italy predisposed Gallienus at least in his favour and he seems to have been that prince's most trusted servant and adviser during the period of his sole reign - 260(?) -268 AD.

Supporting paragraphs: Robert Waterton Henry VI of England
Answer: 1421