Can i increase my height even at the age of 21?

No: I am sorry, but the answer is no. Once we reach skeletal maturity there is no more growth in length of the long bones, which means there is no more increase in height.

Platform shoes: The potential additional length anyone can expect depends on areas in the long bones (arms, legs, fingers)called growth plates. This is where growth occurs and an active open growth plate means more growth is possible. An x-ray of the hands & wrists can be evaluated for bone maturity since there are many growth centers to see. If your bone maturity is > 18yo you are done growing taller.