The current belief is that it can affect the brains of growing children, but they're not sure to what extent. In my state the legal age is 21, the same as for drinking. In adults, straight cannabis will not harm you in any way other than the usual effects of being under the influence and not always making rational choices. Worst case, you sleep it off. Do keep in mind that smoking it can slowly damage your lungs, the same as breathing any find particulates over a long period of time. I personally prefer edibles for long-term use, but the baking process does reduce the potency.
Now, like any drug, the reason you want to take it can be very important. If you need something to escape reality on a permanent basis, obviously you have a larger problem that no drug is ever going to solve. However the occasional evening can be quite entertaining, much like a trip to the water park. Other people (like myself) take it on a regular basis to help with medical problems, usually pain relief or sleep issues, where CBD is the answer rather than the THC component.
For myself, I've had sleeping problems for the past 16 years (since a bad car wreck). I worked with my doctor for awhile trying to find something that would help. The problem is, I'm a computer tech and I need my mind sharp for troubleshooting or writing code. All the prescription drugs leave your brain in a fog through much of the next day, and then what use am I? It turns out that marijuana has a fairly quick drop-off, so it can provide help through the night, and is pretty much out of my system by the time I head for work. As with anything, it takes some time to find the right dose. I take it along with other prescription medications to address different issues, but I've been getting good restful sleep for the past 9 years and feeling much more like myself again. For these purposes I highly recommend it, but talk to someone knowledgeable to guide you to the right strains and give advice on what to expect.