Hi Siberys, long time! So you have started the process of changing gender from male to female. It's usually a long process both physically and psychologically. Good to hear your family and friends are accepting about it - negative attitudes from people around you can often be the most difficult part of transition process.
Like many others here, I also know people who have changed gender. In addition, I have some professional knowledge about transgenderism, partly because it's much more common than most people believe, and partly because I have a colleague who is specialised in research about transgender and transexual persons.
At what age transgender persons come to realise that they want to live as the other sex differs, some have felt that they belong to the other gender since childhood whereas others may realise it much later. I don't think 21 is an early age at all, a majority of transgender persons as well as the rest of us have a clear sexual identity as well as sexual orientation once we've passed puberty and entered young adulthood. Regardless of age, there is always a slight risk that gender changing treatment, including irreversable surgery, will not lead to the positive consequences the person is hoping for. Recent studies however show that the proportion of persons who regret treatment is quite low, around 4%, and that proper assessment and clinical routines minimise the risk for later regret or other problems. Some years ago a study from the Netherlands demonstrated how lack of thorough clinical assessment and counceling and far too short time between seeking health care and surgery led to a large number of patients who regretted the treatment simply because they underwent surgery for other reasons than genuine transgenderism. For example, a significant propotion of patients were actually psychotic and had delusions at the time of surgery, and the short time intervall between seeking sex-changing treatment and actually performing surgery meant that this group of patients did not have time to recover from their delusional ideas.
There is an international "golden standard" of clinical treatment of transgenderism, and in these treatment guidelines, living full time as the desired sex for a period of time before surgery is included. Starting hormone therapy and starting to live as a woman, will over time make you decide whether you want to change sex surgically and legally too, or not. I have no idea what you look like, but I do hope you are not 6'4 and has a chest like a barrel and enormous feet, because it is certainly more difficult to be accepted as a woman in our society if you have very typically masculine physical traits. Also...don't overdo it! Some male to female transgender persons try to compensate masculine physical traits with feminine attributes, especially early on in the transition. They end up looking like drag queens instead of women, and if that is not the aim - don't go there.
Btw, there are also some interesting cultural aspects of transgenderism - in Iran for instance, it's more common with male to female sex changes because homosexuality is a severe sin whereas sex change is viewed as a disorder that can be cured. In Thailand, male to female transgenderism is both more common and more accepted than anywhere else in the world - it is not known exactly why. One man I know went to Thailand to try out living as a woman because he felt it was easier to do it in a more accepting culture.
Good luck with your transition! I hope you will be happy with your choice and that people around you will be accepting and tolerant, as they should be.