It’s age discrimination because if someone’s an adult legally you can’t just say, “your not adult enough sorry”. You should pay attention to their character and how they speak to you more than anything and if you can’t do that for someone who’s under 21 then how are you supposed do that for everyone else?
To answer the question, you can't know for sure if someone is "adult enough" in 5 minutes regardless of their age. Therefore, you have to look at all available information to help you make a decision. How they speak during a short conversation may or may not coincide with their character and maturity. Knowing someone's character takes a lot longer than most people are willing to spend when selling a handgun.
There are exceptions to every "rule." Ignoring the outliers, the majority of people tend to be more mature and become more responsible the older they get, up to a point. Therefore, absent of other compelling information, age can be used as a heuristic for maturity. Is that age discrimination? No, it is not. It's using a known piece of information as a reasonable substitute for an unknown piece of information.
I have no doubt you would like to disagree with me. If you do, please explain how the seller is supposed to go from a complete stranger to knowing your character through a couple of texts or short conversations.