RVN was born 1 July 1976, which means he was 21 years old for the whole of the 1997/98 season.
In the 97/98 season he played for Heerenveen and he scored 13 goals in 31 games in the league, 16 in 40 in all competitions.
This took me about 5 seconds to find out.
Alan Shearer was born 13 August 1970, which means he was 21 for almost the entirety of the 1991/92 season.
In the 91/92 season he played for Southampton and scored 13 goals in 41 games in the league, 21 in 60 in all competitions.
Thierry Henry was born 17 August 1977 which means he was 21 for almost the whole of the season 1998/99 season.
In the 98/99 season he played for both Monaco and Juventus and scored 4 goals in 29 games in the league, 4 in 38 in all competitions.
Dwight Yorke was born 3 November 1971 which means he spent most of his time as a 21 year old in the 1992/93 season, with a small portion at the start of the 1993/94 season.
He was playing for Aston Villa in both seasons. In 92/93 he scored 6 goals in 27 games in the league, 7 in 35 in all competitions, and the 93/94 season scored 2 goals in 12 games in the league and 3 in 14 in all competitions.
There are literally countless examples of players not hitting big numbers until they turn 22/23. The likes of Wright or Vardy weren't even playing professional football at 21.