If the business is charging you less than they would have if you'd told the truth, then they are losing money because you lied to them. You are obtaining something of value at the business's expense under false pretenses, and thus are committing fraud. It's difficult to find a specific criminal statute violated, because most of these cases are civil matters. But in Virginia for example, a liberal reading of §18.2-188 may apply.
It shall be unlawful for any person, with intent to cheat or defraud, to obtain credit at a hotel, motel, campground, boardinghouse, restaurant, eating house or amusement park for food, entertainment or accommodation through any misrepresentation or false statement.
Unless you've racked up $thousands in discounts, the chance of criminal charges is pretty low, but nonzero. Either way, misrepresentation is grounds for terminating the contract allowing you to stay, eat, or whatever. At the very least, you may get a very unfriendly wake-up call and/or some help leaving the establishment. :P
(All this is assuming you succeed, of course. If they don't buy it, then you might be better off.)
Aside from that, the business didn't just take your word for it that you're 70, because it knew some schmuck would lie about their age to get half off. So you almost certainly had to present an ID. If the ID is fake, that is often a crime in itself.