There are two issues.
1) Informed consent. Only with informed consent should re-assignment be allowed. However we should attempt to get a properly informed choice from the 'patient' as early as possible and not assume that there is some pre-determined age prior to which informed consent is impossible.
2) Societal attitudes. Where re-assignment is sought solely because the person cannot cope with their present state within the society we currently have, then there is something wrong with our society and we should fix it.