Who become the oldest atp world number one?
Lleyton Hewitt is both the youngest world No. 1 (20 years, 268 days) and youngest year-end No. 1, while Roger Federer is the oldest No. 1 (36 years, 320 days). Rafael Nadal is the oldest year-end No. 1 (33 years).
Who was the oldest player to become ATP world number 1?
Roger Federer
List of ATP number 1 ranked singles tennis players
Since 1973, 26 men have been ranked No. 1 by the ATP, of which 17 have been year-end No. 1. The current world number one is Rafael Nadal.
Who was the oldest player to become year-end ATP world number 1?
Rafael Nadal
List of ATP number 1 ranked singles tennis players
WTA Rankings
https://en.wikipedia.org/wiki/WTA%20Rankings
List of ATP number 1 ranked singles tennis players
https://en.wikipedia.org/wiki/List%20of%20ATP%20number%201%20ranked%20singles%20tennis%20players
He also became the oldest ATP world No. 1 in 2018 at age 36.
Roger Federer
At 36 years and 195 days of age, he became the oldest ATP world No. 1 by more than three years.
Roger Federer
e set in 2017:[198]

At the age of 33, Nadal was at that time the oldest person to finish as year-end no. 1 player.
Rafael Nadal
Ranking points are awarded according to the stage of tournament reached, and the prestige of the tournament, with the four Grand Slam tournaments awarding the most points. The rankings are updated every Monday, and points are dropped 52 weeks after being awarded
ATP Rankings Year-end No. 1 ranked players
The ATP Rankings are the merit-based method used by the Association of Tennis Professionals (ATP) 
ATP Rankings Year-end No. 1 ranked players
In mid-February, Federer won his third Rotterdam Open
Roger Federer
Roger Federer is the oldest player to ever become ATP world No. 1, while Rafael Nadal is the oldest player to ever become year-end ATP world No. 1. Roger Federer became the oldest ATP world No. 1 in February 2018. At 36 years and 195 days of age, he became the oldest ATP world No. 1 by more than three years. Rafael Nadal became the oldest person to finish as the year-end ATP world No. 1 in 2017 at the age of 33. For ATP Rankings, ranking points are awarded according to the stage of  tournament reached and the prestige of the tournament, with the four Grand Slam tournaments awarding the most points. The rankings are updated every Monday and points are dropped 52 weeks after being awarded.