Whoever willfully or maliciously injures, tears down or destroys any letter box or other receptacle intended or used for the receipt or delivery of mail on any mail route, or breaks open the same or willfully or maliciously injures, defaces or destroys any mail deposited therein, shall be fined under this title or imprisoned not more than three years, or both.
That said, just because it's technically a felony punishable by up to 3 years in federal prison, doesn't mean you'll actually be punished that way. The Department of Justice's Criminal Resource Manual has a section on misdemeanor offenses that apply to postal crime:
Among the misdemeanor dispositions available are: 18 U.S.C. § 1701 (obstruction of mails generally); 18 U.S.C. § 1703(b) (opening,destroying, or detaining mail without authority); 18 U.S.C. § 1707 (theft of property used by postal service); and 18 U.S.C. § 1711 (misappropriation of postal funds). When the charge might best lie under 18 U.S.C. § 1705 (destruction of letter boxes or mail) or 18 U.S.C. § 1706 (injury to mail bags) and in other appropriate circumstances, an applicable misdemeanor may be found in 18 U.S.C. § 641 (theft of government property); or 18 U.S.C. § 1361 (destruction of government property).
That suggests (at least to me) that federal prosecutors are supposed to at least consider misdemeanor instead of felony charges for minor cases of mailbox destruction. Under normal circumstances, destruction of federal property only becomes a felony if the damage (or attempted damage) is more than $1,000.
This is a pretty good example of how US laws are written with prosecutorial discretion in mind. In other systems, the law about destruction of mail or mailboxes would lay out when it's a serious crime and when it's minor. In the US, it's always considered a serious crime because the assumption is that prosecutors won't charge it in minor cases (in fact, official guidance to prosecutors lays out ways they can charge it without charging the felony).
It's also possible to be prosecuted at the state level for this, and if you're actually a juvenile that's much more likely than federal prosecution. The feds don't really like handling juvenile cases; they normally leave those up to the states unless there's some good reason not to. Even if you're an adult, the feds may well leave the issue up to the state for prosecution. At the state level, the threshold for felony vs. misdemeanor vandalism would depend on the state. Whether you're prosecuted at the state or federal levels, there's a decent chance you wind up with at least a misdemeanor on your record. Vandalism is illegal, after all.