This is straight out the Tennessee Wildlife Resource Agency's spring turkey hunting regulations and guidlines:

Measuring a turkey spur: spur length is measured in a straight line from the middle of the spur where it meets the leg to the tip of the spur.

1 year........ 1/2" or less
2 years...... 1/2" to 7/8"
3 years...... 7/8" to 1.0"
4 years + .. 1" or more