A: Generally less than 9mm

An imaging analysis of 240 healthy spines measured the height of the intervertebral disc and compared those measurements against age and gender: “Variation in disc height is determined much more by sex rather than age. The maximum height of the interbody space in the adult lumbar spine was at the L4/5 level (8.9±1.7 mm males, 8.6±1.8 mm females).”

Bach K et al. Morphometric Analysis of Lumbar Intervertebral Disc Height: An Imaging Study. World Neurosurg. 2018 Dec 19. pii: S1878-8750(18)32836-5. doi: 10.1016/j.wneu.2018.12.014. [Epub ahead of print]