Most multicellular organisms are unable to sexually reproduce at birth (or germination), and depending on the species, it may be days, weeks, or years until their bodies are able to do so. Also, certain cues may cause the organism to become sexually mature. They may be external, such as drought, or internal, such as percentage of body fat (such internal cues are not to be confused with hormones which directly produce sexual maturity).
Sexual maturity is brought about by a maturing of the reproductive organs and the production of gametes. It may also be accompanied by a growth spurt or other physical changes which distinguish the immature organism from its adult form. These are termed secondary sex characteristics, and often represent an increase in sexual dimorphism. For example, before puberty, human children have flat chests, but adult females have generally larger breasts than adult males. However, there are exceptions such as obesity and hormone imbalances such as gynecomastia.
After sexual maturity is achieved, it is possible for some organisms to become infertile, or even to change their sex. Some organisms are hermaphrodites and may or may not be able to produce viable offspring. Also, while in many organisms sexual maturity is strongly linked to age, many other factors are involved, and it is possible for some to display most or all of the characteristics of the adult form without being sexually mature. Conversely it is also possible for the "immature" form of an organism to reproduce. This is called progenesis, in which sexual development occurs faster than other physiological development (in contrast, the term neoteny refers to when non-sexual development is slowed - but the result is the same, the retention of juvenile characteristics into adulthood).