If someone is at a bar/restaurant talking to someone, and they accidentally ask them to make plans not knowing they're under age, is there a law that requires them to know their age first (i.e. carding)?
The United States does not recognize the idea of a "date" as an event, and therefore there are no requirements for whom you take or do not take on a date. However, if that date involves drinking, drinking laws apply; if that date involves smoking tobacco products, tobacco laws apply; if that date involves sex, sex laws apply, including and especially those governing consent and a required age for the same. And this makes sense.
Think about it. How easy would it be for two people to spent a few hours getting coffee, seeing a theme park or exploring Paris, wherein one sees it as a date, and the other sees it as two friends hanging out? A "date" is an extraordinarily rough definition of an event, and the sort of thing two people or any number of people could very easily disagree upon, making it very hard to regulate. On the other hand, there's a fairly firm legal definition of what it is to drink and not to drink, to use tobacco or not to use tobacco, and to have sex or not have sex. These things can be clearly delineated; a date on the other hand would just be a pain to lawmakers to try to deal with.
I would however advise you to consider that states have jurisdiction as well, even though their result is the same (no legal definition of date as far as I know).