While researching this topic, I came across all kinds of information but ultimately decided that Nat King Cole summarized it best in the lyric "from one to ninety two"...
In all seriousness, if you don't want to read anymore, the answer is: If you can use a credit card, you can buy CBD. There are no federal guidelines established as of October 1st, 2019 that regulate or restrict CBD purchases to age groups.
Retailers, brands, distributors and obviously dispensaries (which require patrons to be 21+ just to enter) can and do place age restrictions on CBD purchases, but those are solely at the discretion of the retail outlet. Some of them place the age at 18, some at 21.
Why would CBD retailers restrict minors from purchasing?
Assuming that CBD is properly processed and federally compliant, it would contain less than 0.3% THC which would make it non-intoxicating or inebriating. There is very little to no chance of CBD being abused, so it why restrict it to adult use only?
First, every business owner is keenly aware of liability. With risk that there exists an obscure ruling, or law that hasn't been publicized, or simply the chance of an accusation coming down on a business for selling "cannabis" to minors, most (if not all business owners) are very cautious about staying on the right side of the law when it comes to products like CBD. Unfortunately the cannabis stigma and misconceptions about CBD only perpetuate the problem.
Second, very little formal research has been conducted on the long term effects of CBD. Again, this one may be a little bit more about staying on the right side of history, but when you are selling products that don't have decades of clinical studies to support them, it is best to only sell to consenting adults who are capable of making decisions regarding their health.
Next, a massive segment of the CBD industry is vape and smoking related. Depending on the state that you live in the legal age to purchase tobacco and other smoking products like vapes can vary from 18 to 21. In order to stay on the right side of the law, many CBD retailers have opted to just apply their state mandated age restrictions for tobacco products to their CBD offerings.
Lastly: Misconception. To plainly state it, there are many retailers out there who don't understand the laws surrounding CBD. Granted, with the lack of regulation, who can blame them for being confused -- but it is because there is so much mis-information out there that some retailers restrict CBD purchases to 18+ while others believe they have to restrict it to 21+. There are plenty of online resources that state CBD age limits and restrictions and plenty who are stating that there are none. With the proliferation of "fake news" and millions of publications available online ( yes, we know that we are one of those millions of publications) it can be really difficult to sift through fact and fiction.
At the end of the day, the hard answer is: There is no federal guideline on age restrictions for the purchase of hemp derived products including CBD.