It is illegal for a 16 year old to have a sexual relationship with anyone 18 or older. Parent or individual consent is irrelevant because as accurately stated above, you cannot consent to an illegal act.

Is it weird for a 18 year old to date a 16 year old?

Its perfectly legal. First of all, there are no laws governing who people can date. There ARE laws governing who people can have sex with. (If you think that dating someone is just the same thing as having sex with them, then you are too young

Can an 18 year old date a 15 year old in Illinois?

No it isnt illegal unless you have any kind of sexual relationship. Just petting could result in criminal charges for the 18 year old , and landing him on the sex offender registry for life.