diagonal: Youngest National Chess Champions - men
<Niaz Murshed> (*1966) won the championship of Bangladesh at the age of 12 years and 309 days (according to Bill Wall who knows such things). This record stood for a long-time!
In 1978, Murshed finished first in the national championship with two others, but ultimately placed third on a tie-breaker. He went on to win the next four national championships in 1979, 1980, 1981 and 1982. He became the national chess champion again after 30 years in 2012. Murshed is Bangladesh's first ever GM, and the first South Asian Grandmaster, having been awarded the title in 1987.
<Henrique Mecking> (*1952) in 1967 at age of 13 yrs 5mths, Brazil: http://www.brasilbase.pro.br/bca196..., at that time a record, later surpassed / undercut by Niaz Murshed in 1979.
<Bobby Fischer> (*1943-2008) in 1957/8 at age of (near) 14, United States of America.
<Arturo Pomar> (*1931-2016) in 1946 at age of 15, Spain. Sometimes, Arturo Pomar is labelled as youngest national champion, winning the Championship of the Balearic Islands at the age of 11.
<Judit Polgar> (*1976) won the Men’s Hungarian Championship, played in Budapest in December 1991 (42th edition) at age of 15 and 5 months (unbeaten above joint 2nd-3rd Adorjan and Sax, ten players including her sister Zsuzsa Polgar and legendary Lajos Portisch), making her third and final GM norm.
In December 1991, Judit Polgar thus achieved the title of an International Grandmaster of Chess at the age of 15 years, 4 months and 28 days, at the time the youngest player ever to have done so, breaking the record previously held by former World Champion Bobby Fischer.
<Simen Agdestein> (*1967) became Norwegian national champion at age of 15+ in 1982. Agdestein and IM Bjorn Tiller shared first place in the Norwegian championship in the Summer. At the end of December 1982 they played a title match of four games. Agdestein won by 3-1 (+2=2).
<Magnus Carlsen> (*1990) became Norwegian national champion, too, at age of 15+ in 2006. The chess prodigy and his (former) teacher Simen Agdestein, tied for first at the Norwegian Championship in Summer 2006 in a 22-player 9-round Swiss tournament that was held in July 2006, in the city of Moss, just south of the capical of Oslo. To months later, Carlsen won the tie-break play-off to take the title.
<Wesley So> (*1993), Champion of the Philippines, winning the title in 2009 at age of 15yrs 6mths.
<Murray Chandler> (*1960), Co-Champion of New Zealand at age of 15 yrs and 8 mths. The NZL Championships in 1975/76 saw a triple tie for the title (without play-off), after Chandler lost in the final round to William Fairhurst (multiple Scottish Champion), thus allowing record title holder Ortvin Sarapu (born in Estonia, known in New Zealand as “Mr Chess”) and Lev Aptekar (ex-USSR), to catch up. First-timer in alphabetical order: Aptekar, Chandler, and Sarapu.
<Daniel Yanofsky> (*1925-2000) in 1941 at age of 16, Canada.
<Florin Gheorghiu> (*1944) in 1960 at age of 16, Romania.
<Wei Yi> (born 2 June 1999), in end of May 2015, Wei won his first Chinese Championship, breaking the previous national record of <Ding Liren> (born in October 1992, who had won the national title in May 2009 at age of 16 years and 7 months), becoming the youngest Chinese chess champion ever a few days before turning 16 years young.
<Akash G> (*1996, October) is India's youngest-ever national chess champion at 16 years and 14 days. In 2012, G. Akash, a student of computer science at Jawahar Higher Secondary School in Chennai won the National Premier Championship as untitled player.
<Murali Karthikeyan> (*1999, May) in 2015, <Viswanathan Anand> (*1969, December) in 1986, and <Dibyendu Barua> (*1966, October) in 1983 were all at age of 16 to 17.
Note: at the British Championship in Chester 1979, the 14-year-old Nigel Short tied for first place with John Nunn and Robert Bellin, earning his first IM norm, the title was won by Robert Bellin, subsequently Short cannot be counted then as National Champion even some sources do so.
Though some of the child prodigies («Wunderkinder») failed to fully fulfil their early promises (i.e. Niaz Murshed), or / and suffered severe illness (i.e. Pomar, Mecking), they remain strong players.
Other National Champion titles by the mentioned players have been omitted for reasons of consistency and clarity.
<Listing may be incomplete!>