Kohli first played for Delhi Under-15 team in October 2002 in the 2002–03 Polly Umrigar Trophy. He was the leading run-scorer for his team in that tournament with 172 runs at an average of 34.40.[24] He became the captain of the team for the 2003–04 Polly Umrigar Trophy[25] and scored 390 runs in 5 innings at an average of 78 including two centuries and two fifties.[26] In late 2004, he was selected in the Delhi Under-17 team for the 2003–04 Vijay Merchant Trophy. He scored 470 runs in four matches at an average of 117.50 with two hundreds and top-score of 251*.[27] Delhi Under-17s won the 2004–05 Vijay Merchant Trophy in which Kohli finished as the highest run-scorer with 757 runs from 7 matches at an average of 84.11 with two centuries.[28] In February 2006, he made his List A debut for Delhi against Services but did not get to bat.[29]
In July 2006, Kohli was selected in the India Under-19 squad on its tour of England. He averaged 105 in the three-match ODI series against England Under-19s[30] and 49 in the three-match Test series.[31] India Under-19 went on to win both the series. At the conclusion of the tour, the India Under-19 coach Lalchand Rajput was impressed with Kohli and said, "Kohli showed strong technical skills against both pace and spin".[32] In September, the India Under-19 team toured Pakistan. Kohli averaged 58 in the Test series[33] and 41.66 in the ODI series against Pakistan Under-19s.[34]
Kohli made his first-class debut for Delhi against Tamil Nadu in November 2006, at the age of 18, and scored 10 in his debut innings. He came into the spotlight in December when he decided to play for his team against Karnataka on the day after his father's death and went on to score 90.[36] He went directly to the funeral after he was dismissed. Delhi captain Mithun Manhas said, "That is an act of great commitment to the team and his innings turned out to be crucial," while coach Chetan Chauhan lauded Kohli's "attitude and determination."[37] His mother noted that "Virat changed a bit after that day. Overnight he became a much more matured person. He took every match seriously. He hated being on the bench. It's as if his life hinged totally on cricket after that day. Now, he looked like he was chasing his father's dream which was his own too."[15] He scored a total of 257 runs from 6 matches at an average of 36.71 in that season.[38]
In April 2007, he made his Twenty20 debut and finished as the highest run-getter for his team in the Inter-State T20 Championship with 179 runs at an average of 35.80.[39] In July–August 2007, the India Under-19 team toured Sri Lanka. In the triangular series against Sri Lanka Under-19s and Bangladesh Under-19s, Kohli was the second highest run-getter with 146 runs from 5 matches.[40] In the two-match Test series that followed, he scored 244 runs at an average of 122 including a hundred and a fifty.[41]
In February–March 2008, Kohli captained the victorious Indian team at the 2008 ICC Under-19 Cricket World Cup held in Malaysia. Batting at number 3, he scored 235 runs in 6 matches at an average of 47 and finished as the tournament's third-highest run-getter and one of the three batsmen to score a hundred in the tournament.[42] His century (100 runs from 74 balls) against the West Indies Under-19s in the group stage, which was called "the innings of the tournament" by ESPNcricinfo,[43] gave India a 50-run victory and earned Kohli the man of the match. Kohli picked up a leg injury during the match, but recovered in time to play the quarter-final match against England Under-19s.[44] He was instrumental in India's three-wicket semi-final win over New Zealand Under-19s in which he took 2/27 and scored 43 in the tense run-chase and was awarded the man of the match.[45] He scored 19 against South Africa Under-19s in the final which India won by 12 runs (D/L method). ESPNcricinfo commended him for making several tactical bowling changes during the tournament.[43]
Following the Under-19 World Cup, Kohli was bought by the Indian Premier League franchise Royal Challengers Bangalore for $30,000 on a youth contract.[46] In June 2008, Kohli and his Under-19 teammates Pradeep Sangwan and Tanmay Srivastava were awarded the Border-Gavaskar scholarship. The scholarship allowed the three players to train for six weeks at Cricket Australia's Centre of Excellence in Brisbane.[42] In July 2008, he was included in India's 30-man probable squad for the ICC Champions Trophy which was to be held in Pakistan in September 2008.[47] He was also picked in the India Emerging Players squad for the four-team Emerging Players Tournament in Australia. He was in fine form in that tournament and scored 206 runs in six matches at an average of 41.20.[48]