It is not possible to look up living people using a Social Security number. The Social Security numbers of deceased individuals are publicly available on sites that use the government's Social Security Death Index. The cost for this service varies between sites and is free in some instances.
Online services exist to validate Social Security numbers, but these services do not provide identifiable information. Instead, they use patterns found in all Social Security numbers with an issue date prior to 2011 to identify the date and location of issue and to determine whether or not a number is valid.
Twenty-two states ban communication of Social Security numbers to members of the public. Allowing individuals to look up living people by Social Security number is a violation of these state laws.