We use surface gravity to estimate the relative ages of young stars because gravity increases as young stars descend their Hayashi tracks, and gravity is independent of the circumstellar environment or evolutionary models. We have measured surface gravities using high resolution K-band spectroscopy, and extinction using medium resolution near-IR spectra, for 109 low-veiling Class I (LVC1), Class II, and Class III YSOs. We examine how the spectral index, corrected for extinction, evolves with time. Young stars of the same age have a wide range of spectral indices, ranging from Class I to III. Also, YSOs with similar spectral indices can have a wide range of ages, from 0.5 Myr to 20 Myr. After correcting for extinction, LVC1 YSOs have higher spectral indices on average than Class II YSOs, but their spectral index distributions have significant overlap. Dereddening the spectral index also causes some LVC1 YSOs to fall among the Class II objects. While spectral index does decrease with time, the great scatter shows that spectral index is a poor indicator of age. YSOs have a range of spectra index values by time they are ~0.5 Myrs old due to evolution prior to the Class I phase. Ejections may explain optically visible stars that are <1 Myr old, whereas some embedded Class I YSOs are 2 Myr old. Some optically visible Class II and III YSOs are among the youngest stars in this study, allowing visible light diagnostics to be used on some of the youngest stars.