Introduction: It is well-known that cognitive function declines with age. In order to detect changes in cognitive function, cognitive tests should be performed repeatedly. Currently existing cognitive tests come in only a single version, so the subject is likely to remember the contents with repeated testing. And, under the outbreak of coronavirus disease 2019 (COVID-19), in-person assessment should be avoided. This study was performed to develop a new cognitive test (brain assessment, BA) that has 5 versions and can be performed on a personal computer (PC) through the Internet. Materials and Methods: Five thousand subjects performed the online BA, which consisted of 5 subtests: number memory, word memory, mental rotation test, N-back test, and judgment test. We standardized the raw scores (cognitive scores, CSs) using mean and standard deviation, which were 50 and 10, respectively. Then, we calculated the mean CS for each sex and age, plotted the relationships between ages and mean CSs on figures, and calculated the formula of cognitive changes during normal aging. Results: The CSs of all subtests decreased with aging. The regression coefficient was from −0.31 to −0.45. It is noteworthy that in most subtests, the CSs started to increase at 85 years of age. Discussion: Our BA has 5 versions and can be done on a PC using the Internet. We tested the BA in a large number of subjects, and the standard values of CSs were measured in individuals up to 89 years of age. By performing this test repeatedly, subjects can evaluate the degree of their cognitive decline. If the rate of cognitive decline is greater than that predicted using the normalized formula, the subjects can undertake strategies to improve their control of lifestyle-related diseases or other habits of daily living. Conclusion: The BA can be easily taken online using a PC, and its scores linearly declined with normal aging. The BA will be useful for detecting longitudinal cognitive changes and comparing them to the pattern seen in normal aging.