The invention relates to a mobile walking aid (1) having an undercarriage (2), a frame (7; 107; 207; 307) and a support (8) for a person's leg, said undercarriage (2) comprising a two-wheeled front axle (3) and a rear axle (6; 106; 206; 306), wherein the rear axle (6; 106; 206; 306) can be converted between a single-wheeled embodiment with a first wheel (9; 109; 209; 309), and a two-wheeled embodiment. In order to convert the present invention from the single-wheeled embodiment into the two-wheeled embodiment, a conversion kit can be provided which comprises an axle extension (11; 211; 311) that can be mounted on the axle (14; 214; 314) of the first wheel (9; 209; 309), and a second wheel (10; 210; 310), wherein said axle extension (11; 211; 311;) can be mounted, optionally, on both sides of the axle (14; 214; 314) of the first wheel (9; 209; 309) which is arranged laterally and centrally between the front wheels (12, 13).