Short (normal) answer: Yes. It is absolutely illegal.
Lawyerly answer: Yes. It is absolutely illegal. In fact, you will even get on the sexual offender's registry. If a 30 year old was to make out with a 17 year old, he would likely be charged with the misdemeanor known as "annoying a minor," which is codified in P.C 647.6 (a)(1). This section reads: " (1) Every person who annoys or molests any child under 18 years of age shall be punished by a fine not exceeding five thousand dollars ($5,000), by imprisonment in a county jail not exceeding one year, or by both the fine and imprisonment." The State of California defines "annoying or molesting" as "making out" or kissing.
So yes, it is illegal. And it will land you on the sex offender websites as a blue dot. So unless you want to go around to all your neighbors introducing yourself saying "Hi, I am your new neighbor. I am a sex offender, but don't worry, I only make out with 17, almost 18 year olds," you are probably better off just waiting until she (or he) is 18...Doesn't make it less creepy, but at least in the eyes of the law, she'll be legal.
First off, it doesn't make sense to post a question to a we site for lawyers and not want a "lawyerly" answer. That being said,
Your definition of make out is completely lacking. Without any sexual inter course, penetration or oral copulation, it is not illegal for a thirty year old to kiss a minor. BUT, there is a crime in California call "harassing a minor" which can be used to punish the exact type of fact pattern to which you are referring. This is a relatively broad criminal statute that was designed to be a catch-all for all offenses between older and younger persons that did not cross the threshold into sexual relations.
You are not going to get the best answers if in the first sentence of your question you insult the people who volunteer their time to offer help on this site. Lucky you you got two good answers anyway.