A mobile walking aid includes an undercarriage, a frame and a support for a person's leg. The undercarriage comprises a two-wheeled front axle and a rear axle. The rear axle can be converted between a single-wheeled version with a first wheel, and a two-wheeled version. In order to convert from the single-wheeled version to the two-wheeled version, a conversion kit can be provided which comprises an axle extension that can be mounted on the axle of the first wheel, and a second wheel. The axle extension can be mounted, optionally, on both sides of the axle of the first wheel which is arranged laterally and centrally between the front wheels.
