It is, but a Federal law (sponsored as a bill in the House by Foley) makes it a Federal crime to use the internet to solicit sex from a minor, irrespective of local age of consent laws.
(Which leads to the question of whether an e-mail from a husband to his 16 year old wife duely married under the laws of DC or Kansas signed "NORWICH" is a violation of Federal law, but I digress.)
But are these IMs "soliciting sex"? I would think that means asking teens to have sex with you, having found them on the internet. Not using your e-mail to write to people you know.
By that rule, you could get married in DC at age 16, and if you sent your wife an e-mail saying "can we do it tonight"? you could be arrested.....