I think you should be able to discriminate against anyone for any reason, and suffer the market based consequences. They're making a market solution to what THEY perceive is the issue at hand. They're not correct, but at least they are going about it in a free market way instead of lobbying the government to get involved. I personally applaud them for that.
Go back birmingham in the 1950s if you think discrimination is ok. The statute for age discrimination is the same as the statute for race. They can sell or not anything they want, but they are doing something illegal by not selling to anyone under 21