The use of synthetic turf (ST) has become a popular option for many athletic fields. Little is known about how surface hardness and infill depth spatial variability changes over time on third generation (3G) ST athletic fields. A research study was conducted to investigate the impact of field age on surface hardness and infill depth spatial variability from 12 3G ST athletic fields in Tennessee and Georgia (USA) between March 5, 2014 and April 8, 2014. The 3G ST athletic fields tested varied in fiber type including monofilament, slit film, and a combination of monofilament with slit film fibers. All 3G ST athletic fields were directly over gravel with no shockpad present. Surface hardness and infill depth were collected in the same location on all fields. Surface hardness was collected with the Toro Mobile Tester (400–450 samples/field) and infill depth was collected manually with a three-prong infill depth gauge (200–225 samples/field). As field age increased, surface hardness and spatial variability of the surface hardness increased significantly. Surface hardness and infill depth also had a significant negative relationship with one another. The increase in mean surface hardness and variability is partially attributed to infill depth loss and compaction of the remaining infill. Infill depth did not have a significant relationship with age, unless plots were outside the manufacturer’s recommendation for infill depth. Considering the nearly 3000 samples collected in this study, maintaining a minimum infill depth between 30 and 35 mm kept 90% of surface hardness data points below the National Football League limit of 100 Gmax. Results from this study highlight how 3G ST athletic fields can change with age, which may indicate the need for targeted infill applications and decompaction to improve field uniformity over time.