Mean ± standard deviation of directly measured VO2max from the HUNT 3 Fitness Study, a reference population of 3,816 healthy Norwegian adults. Exact figures from Table 2 of the paper cited below. These are averages, not grades: roughly two-thirds of that group fell within one standard deviation of the mean for their age and sex.

Source: Loe H, Rognmo Ø, Saltin B, Wisløff U (2013). Aerobic Capacity Reference Data in 3816 Healthy Men and Women 20–90 Years. PLoS ONE 8(5): e64319, Table 2. Open access under CC BY. doi:10.1371/journal.pone.0064319. This cohort was fitter than North American samples — the authors put the gap at about 20% once body mass is scaled for — so expect to sit lower against this table than against a North American one.