A foldable frame construction for mobility aids has a mounting frame, a pivotal arm, and an operating element. The mounting frame has a fixed end, a limiting end, a limiting hole formed through the mounting frame adjacent to the limiting end, and a through hole formed through the mounting frame between the fixed end and the limiting hole. The pivotal arm is rotatably connected to the mounting frame and has a positioning hole formed through the pivotal arm and selectively aligning with the through hole. The operating element is connected to the pivotal arm and the mounting frame to limit a position of the pivotal arm relative to the mounting frame, and has a fixing ring base securely connected to the pivotal arm around the positioning hole, an engaging bolt movably mounted in the fixing ring base, and a pull button connected to the engaging bolt.