The simple reason is this: puberty is a natural part of the life cycle and attempts to stop it or delay its onset can be risky and aren't to be taken lightly. It will always be a case of slowing onset rather than stopping in any event, as LHRH analogs can decrease the level of FSH and
LH but cannot stop them completely. (Much as some websites talk about "stopping or even reversing" puberty, from a biological perspective this isn't quite correct in terms of what actually happens with the hormones. They're just kept at a low enough level to arrest development) Blocking hormones long-term can also lead to problems with other areas of growth such as the skeleton with osteoporosis and bone wastage, hence my comment that it isn't a course of action to be taken lightly. On a more general level, the question of defining when someone is or isn't mature enough for this level of development is broadly speaking unanswerable - there is considerable development of the brain as well as the rest of the body, so maturity levels will be affected irrespective of prior behaviour. That's why medical intervention is only carried out in extreme circumstances - it's just too much of a grey area to apply any blanket rules.