There is a corruption of morals clause in Pennsylvania’s law regarding consent.
So, in your case, it is definitely illegal for you to have sex with someone 19.
Unlike in most states, in Pennsylvania mistake of age can sometimes be a defense. If you are a victim of sexual assault or rape, contact Rape, Abuse & Incest National Network (RAINN) for online help and local resources.
Pennsylvania's laws regarding the legal age of consent can be confusing.
Below you will find Pennsylvania's statutes (laws) that dictate when various forms of sexual conduct are legal.
Now, as far as your buying that 19 year old a plane ticket, you could be in violation of the Mann Act because you are transporting someone over state lines for the purpose of having sex. The other question you ask is about buying this ticket without permission and I’m going to assume you mean without permission from your parents.
It is not illegal for you to do this if you are using your own money to do so; however, it is certainly unethical on your part, as it does appear you are still living with your parents.
Those who break the law have committed statutory rape.
Statutory rape laws are premised on the assumption that minors are incapable of giving informed consent to sexual activities.