In 1557, 3-year-old Prince Sebastian became the youngest King of Portugal ever.
This is a list of Portuguese monarchs' ages at the time they became kings or reigning queens. For Maria II of Portugal, who was queen during two non-consecutive terms, the age at the start of the first term is given. The median age for a monarch to ascend to the throne is 28 years and 296 days.
^António, Prior of Crato is considered by some historians to be the 18th monarch. From 1580, all other monarchs have the alternative numbering between parenthesis.
^António's precise date of birth is unknown. The dates presented reflect the possible ages between 1 January 1531 and 31 December 1531.
^John VI assumed the regency on 10 February 1792, aged 24y 273d, after his mother Queen Maria I became mad. He was effectively the 22nd oldest person to run the country.
^Afonso III exiled his brother Sancho II to Castile, and assumed the regency on 24 July 1245, aged 35y 80d. He was effectively the 11th oldest person to run the country.
^Peter II de facto removed his brother Afonso VI of the throne, assuming the regency for the rest of his brother's reign. He was 19y 212d, and was effectively the 25th oldest person to run the country.
^The date of his accession in the table is Afonso's declaration of independence after the Battle of Ourique in 1139, and his consequent acclamation. However, Afonso Henriques was the de jure ruler of what is today Portugal when his father Henry, Count of Portugal died on 24 April 1112, when he was only 2y 274d, making him the youngest ruler of the Portuguese territory. His mother declared herself Queen of Portugal and ruled the county until the Battle of São Mamede on 24 July 1128, when Afonso was 18y 365d. If 1128 is accounted for the date of his accession, Afonso Henriques would be the 26th oldest monarch. If 1143, with the Treaty of Zamora, is considered the date of Afonso's accession to the throne he would be the 11th oldest monarch.
^John's father Afonso V abdicated briefly on 11 November 1477, and John consequently became king aged 22y 192d. Afonso would later return to the throne 4 days later. If 1477 is accounted as the date of John's accession, then he would be the 22nd oldest monarch.
^After his brother Pedro IV's abdication, and the appointment of his niece Maria II to the throne, Miguel assumed the regency on 2 May 1826. He was 23y 188d, and was effectively the 23rd oldest monarch. He would later proclaim himself king 2 years later.
^Maria II succeeded her father Pedro IV of Portugal, who was also Emperor of the recently independent Brazil, after his abdication on 2 May 1826. However, she never set foot in mainland Portugal, and her uncle Miguel usurped the throne and triggered a civil war. With the end of the war, Maria was again acclaimed queen on 26 May 1834, aged 15y 52d. She would be the 31st oldest monarch.