what if the girl lied about her age?

We could find no provisions in Maryland statutes that allows an affirmative defense based on general lack of knowledge of the age of a minor.

§ 3-324. Sexual solicitation of minor.
(a) "Solicit" defined.- In this section, "solicit" means to command, authorize, urge, entice, request, or advise a person by any means, including:
(1) in person;
(2) through an agent or agency;
(3) over the telephone;
(4) through any print medium;
(5) by mail;
(6) by computer or Internet; or
(7) by any other electronic means.
(b) Prohibited.- A person may not, with the intent to commit a violation of § 3-304, § 3-306, or § 3-307 of this subtitle, knowingly solicit a minor, or a law enforcement officer posing as a minor, to engage in activities that would be unlawful for the person to engage in under § 3-304, § 3-306, or § 3-307 of this subtitle.
(c) Jurisdiction.- A violation of this section is considered to be committed in the State for purposes of determining jurisdiction if the solicitation:
(1) originated in the State; or
(2) is received in the State.
(d) Penalty.- A person who violates this section is guilty of a felony and on conviction is subject to imprisonment not exceeding 10 years or a fine not exceeding $25,000 or both.