If a women is 26 and she is dating someone who is 17 is that illegal?

Well it is a pretty disgustingly large age difference.  In most of North America if they have sex it would be considered statutory rape as the 17 year old cannot give consent to someone more than 3 years above his age.  Also, just a random stab in the dark here, is the 26 year old in some position of power over the 17 year old?  I'm putting my money on a coach or managerial position.

Not in most countries, where 16 is the age of majority and you can date anyone you want. America tends to be less civilised about these things, they don't even trust you to have a proper drink until you're 21! It's OK for the government to send you off to get shot at and blown up, however...

Depends in which state or country