Yes, the age things are kinda spread out. If you are willing to start a new game, I have modified a bunch of the starting config files to make the high school students 14-19 and adults start at 20. I changed the ages of all the special character students and made a few of them more petite. There is a world creation setting for adult ages that sets a limit of how young they can be if they have children, currently they can get "pregnant" at age 14 so the youngest mom would be 32 with the default student age of 18. I changed Suzi and Stacy Slutsky's ages to 14 and 28. I also take 10 years off my male principal's age, making him 26. I changed the ratio of male to female students a bit so you start with more female students.Neither of the files at these locations in my game has anything about age, min or otherwise, in them.
I built a 7z file with all my changes. you should be able to drop this in your main game folder and have it overwrite all the files. It is just a bunch of xml files so you can go in and look at what I did and make whatever changes you want.
I included the hhs+.exe.config file where I changed the date and money locale from en-GB to en-US but a lot of that seems to have been hard coded as I still get "meeting will be at sixteen" kind of dialog. Maybe barteke22 can enlighten us on what he did there.
I also included the NormalSchool.xml even though I made no changes to it. I believe you can change your starting amount of money and reputation in that file if you want to make the early game a little easier.
As always, back up anything you replace in case this goes bad. After modifying these files or just replacing the original files with my modifications, you will have to start a new game to generate a new population with the changed values.