"Almost" does not count in most human activities, and especially not in politics. So, if the right to vote is only when you are 18 years old, then that right does not exist for anybody who is 17-going-on-18, i.e., almost 18 years old.
Passage of Proposition 18 would allow a 17-year-old to participate in primaries and any other elections held during the Election Year if they can prove that they will be 18 by the time of the General Election. In California since the primaries are held approximately eight months before the final election, somebody of age seventeen and four months could vote. One major problem has to do with the effect on Primary Elections. Very simply, 17-year-old youngsters in California have been exposed during their formative years to a most liberal school system, so they have not had a chance to gain much experience beyond the education system. Youngsters at that age are engaged in gaining experience to develop the skills necessary to evaluate complex political issues. We should not be burdening these young people to make fundamental decisions that may have a profound effect on their future as well as the rest of the society.
Consider some specific issues regarding our own election process. The California election system has what is called the top two primary system. That is, regardless of how many candidates are competing for a given position such as Assembly person or State Senator, only the top two vote getters in the primary election can make into the General Election. If a 17-year-old gets a chance to vote, that would certainly bias the top-two system, favoring the most liberal candidates.