The US statutory ban for overstaying, or "unlawful presence" as it is called, is codified at 8 USC 1182(a)(9)(B):
(B) Aliens unlawfully present
(i) In general Any alien (other than an alien lawfully admitted for permanent residence) who—
(I) was unlawfully present in the United States for a period of more than 180 days but less than 1 year, voluntarily departed the United States (whether or not pursuant to section 1254a(e)  of this title) prior to the commencement of proceedings under section 1225(b)(1) of this title or section 1229a of this title, and again seeks admission within 3 years of the date of such alien’s departure or removal, or
(II) has been unlawfully present in the United States for one year or more, and who again seeks admission within 10 years of the date of such alien’s departure or removal from the United States,
Note that there is no ban for those whose unlawful presence totals less than 180 days. Furthermore, unlawful presence does not accrue for minors: at (a)(9)(B)(iii)(I), there is an exception excluding time before the 18th birthday:
No period of time in which an alien is under 18 years of age shall be taken into account in determining the period of unlawful presence in the United States under clause (i).
Therefore, the number of days you have accumulated is the period between your 18th birthday and your day of departure. If you left three months after your birthday, this number is well below 180, so you do not trigger the provisions of (a)(B)(i).
Of course, the fact that you do not have a statutory ban does not mean that it will be easy for you to enter the US. But if you find sympathetic visa and immigration officers, the fact that you left fairly promptly after turning 18 could work in your favor.
The chance of being granted admission will of course increase as you remain outside the US for longer periods. If you left the US recently, the chance that they'll suspect you're trying to return to resume your life in the US will be stronger. Regardless of how long ago you left, your application to enter the US will be much more likely to succeed if you include (in your visa application) or are prepared to show (at the border) good evidence of strong ties to your country of residence, particularly compelling plans for what you intend to do there after the end of your summer vacation.