Is talking to a 17 year old online illegal?

Under California Penal Code Section 288.4, it is illegal to arrange a meeting with a minor or a person you believe to be under the age of 18 for the purpose of lewd and lascivious sexual conduct. This includes using means such as online chat rooms, Facebook, Twitter and Instagram to contact the minor.

Is it illegal for a 17 to date a 24 year old UK?

In the UK there is no law stating it is illegal for a 23 year old to date a 17 year old. As you are 17 and a young adult, you are old enough to decide if you want to be in a relationship with this man. Its important that both partners consent to the relationship, and that they trust and respect one another.