Hello student you need not to get tensed because of these things. Start preparing for the examination you're concerned about don't listen to friends if they don't support you and if they mock you about your heights ignore them and in case if it's intolerance please complain against them to their parents or in nearby police station.

Age 21 is not bad to start preparing for SBI examination as you need to be at least graduate to appear in SBI examination and by the time you graduate you must be at least 20-21 year old, so stop being so negative stay in a positive atmosphere it's very important to achieve anything.

Believe In yourself no matter how much I motivate you at the end of day its you who need to push your self and work hard to achieve your dream. Read biographies/inspirational books.

Feel free to comment if you've any doubt

Good luck