Sixteen-year-old Americans cannot vote, aside from school president or classmate Most Likely To Succeed.
They cannot marry or obtain a passport without parental permission. They cannot purchase a lottery ticket.
Nor can a sweet 16, of any nationality, join the US-based LPGA Tour full-time.
The policy is wise, and not because the "L" stands for Ladies.
The demands of a tour golfer far exceed negotiating 72 holes per week: pro-am tournaments, schmoozing with sponsors, accommodating fans, dealing with the media. Most significantly, it is facing athletes old enough to be their mother.
It is an equally sound policy to grant exceptions, as long as it is exercised selectively.
Mike Whan, the LPGA commissioner, did so last week for Lexi Thompson, a 16 year old who plays and conducts herself like a 20-something. Close the book on the 2012 Rookie of the Year competition.
Whan has exposed himself to charges of exploitation. He knows Thompson will bring instant attention to women's golf, which is getting squeezed on the crowded sports landscape, more than any newcomer since Michelle Wie.
Whan did not arrive at this decision lightly. The commissioner expanded his evaluation of Thompson beyond the course ropes. Otherwise, he easily could have been fooled.
Thompson, at 6ft, towers over nearly all of her new peers. She hits it into the next time zone, having outdriven as well as outscored all her elders in winning the Navistar Classic last month.
Whan made like paparazzi (without camera and notepad) in observing Thompson off the course to gauge her maturity.
He liked what he saw. Example: as the ink was drying on her Navistar event cheque of US$195,000 (Dh715,650), Thompson announced that she would peel off $25,000 for Wounded Warriors, which aids the families of injured soldiers.
He noticed that expert counsel is a text message away - from big brother Nick, a four-year PGA Tour player.
Waiving the rule for the most capable of teenagers should not - and, under Whan, will not - become routine. While he has welcomed a few almost-18s into the fold, Whan claims to have rejected about 50 applicants.
Thompson's transition arrives on the heels of another age-related milestone in American women's sports.
Happy 30th birthday to Serena Williams.
She and Thompson may hail from contrasting backgrounds and engage in different games. But, seeing as how Thompson's powerful stroke could elevate her sport to a new level just as Williams did with her megaton swing, the youngster might learn from the virtues and vices exhibited by the premier tennis player of this generation.
With guidance from her father and big sister Venus, Serena has done it her way.
She never held a total devotion to tennis, which has sapped the drive of others in their 20s, if not driving them out entirely. Pouring her soul into designated tournaments, while disappearing from the scene in between, is an approach that might not work for others. For her, it has been a straight-sets winner. What were once thought to be potential distractions - fashion, business, charitable endeavours - instead have brought balance to her identity.
Tennis is treated seriously but not as life or death. While others burn out, Williams keeps the pilot light on low, except for the nine or so tournaments she enters annually.
Thompson can also benefit from Williams's failings.
Two years ago, in a profane rant, Williams threatened to cram the ball down a match official's gullet for penalising her on match point for a foot fault. The official's call might have been harsh, but still.
Her boorish behaviour resumed at the US Open this summer, when a referee docked Williams for shouting before a point was completed. Again, the referee might have more fairly ordered a replay of the point; Williams, after all, merely blurted "C'mon."
No matter the content of her yell, Williams did violate a rule. Yet, she berated the arbiter mercilessly for the remainder of the match, though it might be a small measure of growth that her PG-rated diatribe did not force the television network to press the bleep button.
Granted, such conduct by male players is often cast as admirable feistiness (see McEnroe, John). Women athletes are held to a different standard, as Williams has discovered from the backlash.
More egregious, Williams has a habit of stringing tournaments along, either refusing to commit until late or withdrawing with flimsy (or no) excuses. The WTA, which, would crack down on lesser players, must grin and bear it with the tour's leading lady.
These circumstances might not apply to golf, but the teenage prodigy can profit from observing the choices, smart and dumb, made by the just-turned-30 virtuoso.
By borrowing from some and rejecting others, the girl too young to buy a lottery ticket can hit the jackpot, get her parents-approved passport stamped from playing tournaments worldwide, turn down Twittered marriage proposals from new fans and answer her calling to become the Tour's Most Likely To Succeed.