I'm 17 about to turn 18 and am dating a 26 year old and my family is threatening to call the cops on him. Is it illegal? Can he go to jail because of me?

Depends on where you live.

Also depends on what you did with him. If you did anything sexual there will be a problem. If not he is in the free and clear.

Nowhere is there a legal minimum age for dating. However, having sex with someone under the age of consent is illegal. The age of consent does vary but most places are at 16.

If you are soon to be 18, then cool it until you reach that milestone. If he cares about you (and his freedom and reputation), he will wait.

I must say, though, that its an unusual situation where a 26 yr old many wants to be involved, even with an 18 yr old. I can understand your parent's concern. In the vast majority of these cases the attraction is sexual. So asking him to wait is a good test of the depth of his feelings for you.

In Canada, the age of consent is 16, I believe it is the same among most of the Western world.

If you are having sex, then it will depend on the state and the age of consent in your state.
But also your parents have many other laws, if they order him not to take you anywhere, you are a minor, so he could be "interfering with custody" if he takes you anywhere against their wishes. It would be hard in some places, but if the parents are well respected, if they have pull with the courts, yep the DA would find some things to at least keep him in jail for a few days until bail is arranged and can even latter drop charges.

A 26 year old dating a 17 just sounds a little creepy.
If she was 27 and he was 36 it all changes, as they both age.

Sounds like a child preditor. Does he know your real age ?

Ask him to wait until you are 18. Your parents are in the driver's seat until then.