Gukesh is NOT the Youngest World Champion, says Topalov
Former World Champion, Veselin Topalov took to Instagram throwing light on the fact that Gukesh D is not the Youngest Ever World Champion but Ukrainian GM Ruslan Ponomariov is!

Ruslan Ponomariov was indeed the Youngest FIDE World Champion who won the title at 18 years, 3 months and 12 days! But he wasn’t the “Undisputed” World Champion. Let’s take a trip down memory lane to understand what this means.
In 1993, Garry Kasparov formed a separate organization called “Professional Chess Association” after his dissatisfaction with FIDE’s functioning in organizing his match against Nigel Short. From that point forth, there stood TWO World Champions with separate World Championship cycles! During that year, Kasparov was PCA’s World Champion while Karpov won the FIDE World Championship after beating Jan Timman. In 1998, FIDE decided to strip off the Interzonal and Candidates system but instead introduced a Knockout Tournament system for the World Championship. The first edition was won by Karpov, who was directly seeded into the finals but later resigned his title the next year in protest of the new rules. The knockout system wasn’t received with open arms by the other top players who decided against participating in the event itself. This allowed Alexander Khalifman and Viswanathan Anand to take the titles in 1999 and 2000, while Kramnik upset Kasparov in the separate cycle of WCC called “Braingames World Chess Championship”, later referred to as the “Classical World Championship” by Kramnik.
In the year 2002, Ruslan Ponomariov, in exceptional fashion became the FIDE World Champion after winning the 128-player knockout tournament. There was no unification of FIDE and the Kasparov cycle at the time which led to confusion in deciding and validating a World Champion. It was only in 2006 when the titles were reunified through the Kramnik-Topalov match to crown an “Undisputed” World Champion. Although Ponomariov, Khalifman, Anand (2000), etc. won the FIDE World Championship titles before 2006, they are not considered to be “absolute”.

So, Gukesh D as stated by FIDE stands to be the Youngest “Undisputed” FIDE World Champion!