At what level of use? And for which demographic groups?
That depends on which studies you read, and which aspects of marijuana use they are reporting.
Some of the effects, notably the precipitation of psychotic disorder (related to schizophrenia), act on a developing brain but not a mature brain. Thus, this effect is a danger for users under the age of 25 years old. I believe that this effect follows a classic dose-response pattern, in that higher levels of use equate to greater risk.
Other effects, such as the loss of initiative and motivation, seem to affect older people as well as younger people. This is because some of the components of marijuana cause the death of certain cell types which are crucial for these functions. I think that this effect was observed even in what the researchers in one study called "low use" groups--those who used marijuana about once a week. The deficits in motivation and initiative were observed three months after the last use of marijuana. Thus, the effect can have a long duration, but this study did not establish whether the effect eventually dissipates and if so, after how long.
Other researchers are looking at other demographics and other patterns of marijuana use. I've seen several studies described that looked at the effects in pregnant women, but have not read them that closely to know what the effects on the developing baby are.