Here is a comparison of stats between 3 Barca players before they turned 18. Messi made only 9 appearances and scored 1 goal. Fati is the youngest debutant. Bojan Krkic has most goal involvements.
Stats before they turned 18:
Lionel Messi
9 games, 1 goal
Debut - 17 years/3 months/23 days
Ansu Fati
20 games, 5 goals, 1 assist
Debut - 16 years/9 months/26 days
Bojan Krkic
48 games, 12 goals, 6 assists
Debut - 17 years/0 months/20 days