The #1 single on the Billboard Hot 100 is Lorde "Royals". She doesn't turn 17 until November 7th, and that means she is the youngest artist to score a #1 single since Tiffany, who was 16 years old when "I Think We're Alone Now" hit #1 in November of 1987.
Stevie Wonder was the youngest singer to have a #1 single. He was just 13 years old when "Fingertips (Part 2)" topped the chart in 1963.
The youngest woman was Peggy March, who was 15 years and one month old when "I Will Follow Him" hit #1. That was also in 1963.