I found on Jayski.com a list of Young NASCAR drivers. Unbelievable at the Ages. Joey Lagano may be young but not the Youngest.

Logano young, but not the youngest: #96-Joey Logano is young, but at 18 years, 3 months, 20 days, he'll not be even close to the youngest to make his Sprint Cup debut when he makes the big jump Sunday at New Hampshire. Driver, debut date and track listed with their ages:
Tommie Elliott Aug. 1, 1951 Altamont-Schenectady Fairgrounds 15 years, 7 months, 5 days
Darryl Sage July 10, 1982 Nashville Speedway 17 years, 2 months, 2 days
Bill Morton Oct. 9, 1955 Memphis-Arkansas Speedway 17 years, 2 months, 17 days
Donald Thomas May 21, 1950 Martinsville Speedway 17 years, 10 months, 11 days
Bobby Hillin Jr. April 18, 1982 North Wilkesboro Speedway 17 years, 10 months, 13 days
Chuck Bown Jan. 23, 1972 Riverside International Raceway 17 years, 11 months, 1 day
Jeff McDuffie April 20, 1980 North Wilkesboro Speedway 18 years, 0 months, 20 days
Joey Arrington Sept. 8, 1974 Richmond International Raceway 18 years, 1 months, 14 days
Buddy Baker April 4, 1959 Columbia Speedway 18 years, 2 months, 10 days
Joe Ruttman Jan. 20, 1963 Riverside International Raceway 18 years, 2 months, 23 days