Tell them upfront and honestly, it was their daughter who lied to them not you. If they hold it against you than they're unreasonable. They CAN deny you access to their daughter but they could do that if they thought you were 19, seeing as how they didn't another 2 years shouldn't mess them up terribly. Explain everything, how you met their daughter, what your intentions are, everything. Be upfront and forthcoming and I seriously doubt they'll have problems with you.
If they do have problems with you I'm sorry to say that her family may not be the best for you to become involved with anyway.