I don't know your state's laws, but I would start by seeing what their laws are concerning intrastate PRIVATE SALES. I am a FL resident, and in my great state I was able to purchase a handgun at 18 via private sale.
Next, see if OC requires a permit. If it doesn't, see if it has an age requirement. If neither of those exist (and OC is generally legal), 18 is probably the age limit for OC of any kind.
Although it may seem irrelevant, some states (such as Ohio) have laws that prohibit handgun purchases to anyone under 21 period (including private sales) but allow OC without a permit and no age limit (besides federal 18). Although it's easy to see the spirit of Ohio's laws, an 18 year old FL resident who obtained their handgun legally (FL resident to FL resident private sale made inside FL's borders) can handgun OC in Ohio at 18 without a permit, whereas an 18 year Ohio resident can't generally handgun OC at 18 unless they somehow got the handgun legally (which is generally prohibited by their state laws...not sure about exceptions military/LEO/state swapping resident with gun from old state).
The point of my post is to encourage you to keep looking-just know that it may seem extremely complex. This is an age group that most people don't care about when it comes to rights, so cops and gun store owners alike tend to blow you off. In my state I tried buying a handgun from a private sale at a gun show but the seller wasn't sure so he asked a cop to come over and he ignorantly said it was illegal. Keep looking for your answers...I'm sure someone has them.
Best wishes from a fellow under-21er,