Between the 34 states and Washington, D., C. Under marijuana growing laws, you must be 21 years old to grow cannabis plants in any state in the U.S. UU. You can only grow a certain number of plants or have a designated area in some states where you can grow marijuana for recreation.
In California, you can only grow 6 cannabis plants per adult 21 and older for recreational and medical use. It's legal to grow recreational or medicinal products for personal use, but there are restrictions. You can't grow your own cannabis and start selling it to family or friends, that's a big no-no. However, you can grow up to 28.5 grams of cannabis for personal use, but you can't take that amount away from home.
You can also grow up to six plants on your property, but it's best to grow it indoors, because it's illegal to grow cannabis in an area visible to the public. See this link for more detailed information on California's cannabis cultivation laws. Coloradans can grow marijuana in their homes for personal use.