In the species Homo Sapiens (thinking man, believe it or not), the gestation period following conception (the result of sex, recreational or otherwise) is approximately nine months. After twenty more years, physical maturity has usually set in. Psychological maturity is another mater entirely. This is actually the major function of parents in Homo Sapiens.
Or at least it is supposed to be. See discussions attached to the entry for "family". The parents in most human societies are charged with the task of causing psychological maturity in their offspring. They no longer seem to be held accountable for it.