PP have brought up much more important points than this, but I do want to point out that not everyone who attends your in-state colleges also attended K-12 schools in the same state, so there very well may be other incoming freshmen who attended school under the September birthday cutoff most common in the rest of the country (about 2/3s of the states during the time frame when you would have been entering kindergarten). Also, other countries have different practices for school entry and for how long pre-college education (and other activities, such as compulsory military service) last.
(And there have been a number of charming little news stories over the years about colleges graduating a very young student and a very mature student in the same class. A milder version of this story happens every year, in nearly every university, to much less fanfare.)
On another note, I've been reflecting about some of our graduating seniors, and a few in particular come to mind: over the course of this year, I've worked with a handful of 12th graders who could be described as having some level of giftedness in some domain of intellect, ranging from moderately to profoundly gifted. (All twice exceptional, of course, given my day job.) They are heading off to a range of post-secondary options, not necessarily aligned with conventional expectations for intellectually gifted learners, but in accordance with their choices and who they are holistically. Some are already nineteen, one turned seventeen this school year, and the remainder are the more-expected eighteen. The older ones may have been retained, or held out/red-shirted, sometimes at much later grades than kindergarten. At no point during their high school career has age been the most important way anyone has looked at them, and now that they are moving on into adulthood, it will become even less important. None of the 19-year-olds have had obstacles related to their age while here, and actually struck me in our final interviews as being noticeably more mature and thoughtful about next steps in life than many of their classmates--something which I believe these particular young people will use productively in their post-secondary lives.
I'm not a fan of either retention or red-shirting on general principle (we can talk about the research/group data on another day), but ultimately these kinds of placement decisions are what you make of them--initially how parents communicate the decision to their children, and eventually how young people choose to frame their own lives moving forward. I don't know your parents, but as a parent, and someone who's worked with a great many parents of school-age children, I can say that most parents do the best they can for their children with what they know at the time. We're often wrong, but usually not ill-intentioned. Whatever the situation with your family, the next stage of life is going to be almost entirely about your choices, and what you do to find your fit as a whole person, not just on the basis of a number that says almost nothing meaningful about who you are essentially.
