Grass spiders (Araneae: Agelenidae) make flat, non-sticky webs that extend outward from a funnel. To capture prey, spiders hide in the funnel and when a small arthropod lands or falls onto the webs’ surface, they run out of the funnel and bite the prey before the prey can walk or fly off the web surface. These webs can last weeks or months and the spiders continually add silk to them. We tested the hypotheses that prey capture performance would improve as the webs increased in thickness, and attempted to estimate optimal web age for prey capture purposes. We collected our local grass spider species (Agelenopsis pennsylvanica) and had them to build webs in optically clear plastic boxes, documenting web construction over the course of several weeks. Experimentally, we presented the spiders with prey items, and used high-speed video cameras (500 frames per second) to record their prey-capture behaviors. These included sensing, pursuing, and biting prey. As reported previously, the spiders frequently stop and reorient themselves while approaching prey (mean ± s.d.= 2.2 ± 1.6 stops per run). These corrections suggest that the vibrations are important for locating prey, but the system is imperfect. However, the spiders were equally likely to catch prey on webs of a wide range of ages (linear regression p = 0.45). Age of the webs had no impact on success rates of prey capture, nor did it impact maximum speed during prey approach (37 ± 15 body lengths/s), number of stops, nor magnitude of reorientation required to catch prey. Remarkably, for a web that is commonly built and reinforced over several weeks, the spiders could catch prey within one hour of commencing web construction, on a web consisting only of a small number of nearly-invisible threads.