If someone's had low self esteem all their life, is it possible for them to make a 180º turn and change that aspect about them in their early 20's?

It’s possible in your 20s, if you have the right people and the right environment to thrive rather than survive, and nothing awful happens to shake your world. There is still hope. It never happened for me, and in fact got worse as the decades past but that’s because I had several “shake my world” events to cope with, and I was in an unloving and unkind environment with the wrong person/people most of my life. The kind that tear you down rather than build you up. Don’t be with those people. It will destroy your soul. Good luck!