Yes, this would be "illegal" sexual activity. The 17 year old can be charged as an adult.

(720 ILCS 5/11-9.1) (from Ch. 38, par. 11-9.1)Sec. 11-9.1. Sexual exploitation of a child.

The legal statutory age of consent to have sexual intercourse in the state of Illinois is 17. If a pregnancy occurs and it is determined that the minor was under the age of 17 when the child was conceived, there may be criminal action taken against the biological father. It would be up to the prosecutor to determine whether charges will be filed. This may be a misdemeanor or a felony charge depending on the situation and past criminal history. Misdemeanors carry a maximum punishment of a fine and less than one year of incarceration. Crimes for which a person may be punished by more than one year in jail or in prison are categorized in Illinois as felonies

In my interpretation of the law below, I would say kissing is not illegal in this situation, but could be risky if it were to lead to something more. Sec. 11‑9.1. Sexual exploitation of a child. (a) Any person commits sexual exploitation of a child if in the presence of a child and with intent or knowledge that a child would view his or her acts, that person: (1) engages in a sexual act; or (2) exposes his or her sex organs, anus or breast for the purpose of sexual arousal or gratification of such person or the child. (a‑5) A person commits sexual exploitation of a child who knowingly entices, coerces, or persuades a child to remove the child's clothing for the purpose of sexual arousal or gratification of the person or the child, or both. (b) Definitions. As used in this Section: "Sexual act" means masturbation, sexual conduct or sexual penetration as defined in Section 12‑12 of this Code. "Sex offense" means any violation of Article 11 of this Code or a violation of Section 12‑13, 12‑14, 12‑14.1, 12‑15, 12‑16, or 12‑16.2 of this Code. "Child" means a person under 17 years of age. (c) Sentence. (1) Sexual exploitation of a child is a Class A misdemeanor. A second or subsequent violation of this Section or a substantially similar law of another state is a Class 4 felony. (2) Sexual exploitation of a child is a Class 4 felony if the person has been previously convicted of a sex offense.

Yes this would be illegal. 17 is the legal age to consent to have sexual contact in Illinois. In this situation, the 17 year old is at risk of charges and arrest. Also, at age 17, sexual offense cases can be moved to adult court. This is up to the judge. (720 ILCS 5/11‑9.1) (from Ch. 38, par. 11‑9.1) Sec. 11‑9.1. Sexual exploitation of a child. (a) Any person commits sexual exploitation of a child if in the presence of a child and with intent or knowledge that a child would view his or her acts, that person: (1) engages in a sexual act; or (2) exposes his or her sex organs, anus or breast for the purpose of sexual arousal or gratification of such person or the child. (a‑5) A person commits sexual exploitation of a child who knowingly entices, coerces, or persuades a child to remove the child's clothing for the purpose of sexual arousal or gratification of the person or the child, or both. (b) Definitions. As used in this Section: "Sexual act" means masturbation, sexual conduct or sexual penetration as defined in Section 12‑12 of this Code. "Sex offense" means any violation of Article 11 of this Code or a violation of Section 12‑13, 12‑14, 12‑14.1, 12‑15, 12‑16, or 12‑16.2 of this Code. "Child" means a person under 17 years of age. (c) Sentence. (1) Sexual exploitation of a child is a Class A misdemeanor. A second or subsequent violation of this Section or a substantially similar law of another state is a Class 4 felony. (2) Sexual exploitation of a child is a Class 4 felony if the person has been previously convicted of a sex offense. Source: P.A. 94‑140, eff. 7‑7‑05

If there isn't any physical sexual activity going on, there are no laws prohibiting two people to date. However, any sexual activity, including fondling breasts, touching of sexual organs, etc., between a person 13-17 and a person more than 5 years older, is considered "Aggravated Criminal Sexual Abuse," a Class 2 felony. This is punishable by 3-7 years in prison and up to a $25,000 fine. One thing to note as well, a person 13-17 engaging in sexual actvity with a person less than 5 years older is "Criminal Sexual Abuse," a Class A misdemeanor.