Hello from the fourth Charles/Liza shipper!
I agree with you completely. That was the best moment of the season (for us), and unlike some other people here, I don't see that many problems with this relationship.
By telling her this to her, and using the verb "dating", he obviously wants something longterm with her, so I don't think (or hope) that this is just going to escalate on the basis of sexual tension and then become an awkward avoiding at the workplace. I really think there is some potential in this relationship as they seem to have similar interests and understanding for each other, not just physical affection. Him finding out about her real age will only be a relief if he doesn't already know.
I hope the writers will give them proper space to develop a relationship.