An infant carrier apparatus comprising: a support frame including a seat assembly that has a longitudinal axis extending from a front end to a rear end thereof, and a backrest assembly including a backrest frame and at least a holder socket, the backrest frame being mounted with the support frame via the holder socket, and the holder socket being adjustable along the longitudinal axis between a first position and a second position, the support frame further includes at least a guide rail, and the holder socket is movable along the guide rail substantially parallel to the longitudinal axis, and the holder socket has an actuator portion (150A, 150B), the actuator portion (150A, 150B) being movable with the holder socket along the longitudinal axis relative to the support frame; characterized in that: a restraint element (154A, 154B) is arranged at a fixed location on the support frame, wherein the restraint element (154A, 154B) is movable to engage with the holder socket when the holder socket is in the first position to block displacement of the holder socket from the first position toward the second position, and the actuator portion (150A, 150B) is operable to push against the restraint element (154A, 154B) while the holder socket is in the first position so that the restraint element (154A, 154B) moves to disengage from the holder socket for allowing displacement of the holder socket from the first position toward the second position. a support frame including a seat assembly that has a longitudinal axis extending from a front end to a rear end thereof, and a backrest assembly including a backrest frame and at least a holder socket, the backrest frame being mounted with the support frame via the holder socket, and the holder socket being adjustable along the longitudinal axis between a first position and a second position, the support frame further includes at least a guide rail, and the holder socket is movable along the guide rail substantially parallel to the longitudinal axis, and the holder socket has an actuator portion (150A, 150B), the actuator portion (150A, 150B) being movable with the holder socket along the longitudinal axis relative to the support frame; characterized in that: a restraint element (154A, 154B) is arranged at a fixed location on the support frame, wherein the restraint element (154A, 154B) is movable to engage with the holder socket when the holder socket is in the first position to block displacement of the holder socket from the first position toward the second position, and the actuator portion (150A, 150B) is operable to push against the restraint element (154A, 154B) while the holder socket is in the first position so that the restraint element (154A, 154B) moves to disengage from the holder socket for allowing displacement of the holder socket from the first position toward the second position. a restraint element (154A, 154B) is arranged at a fixed location on the support frame, wherein the restraint element (154A, 154B) is movable to engage with the holder socket when the holder socket is in the first position to block displacement of the holder socket from the first position toward the second position, and the actuator portion (150A, 150B) is operable to push against the restraint element (154A, 154B) while the holder socket is in the first position so that the restraint element (154A, 154B) moves to disengage from the holder socket for allowing displacement of the holder socket from the first position toward the second position.