A boy becomes Bar Mitzva on one of the nights of Chanuka. If he lights the Menorah at the preferred time (appx 15-20 minutes after Shkia to the best of my knowledge) it is not Tzeis HaKochavim yet. Would it be better to wait until Tzeis the night he becomes Bar Mitzva and lose out the Pirsumei Nisa of people still on the street or would it be better to wait and do the Mitzva till Tzeis HaKochavim when he is definite Bar Mitzva?
Quoting from נטעי גבריאל: הלכות והליכות בר מצוה והנחת תפילין,
נעשה בר מצוה בחנוכה רשאי להדליק נ"ח מבעו"י להנוהגים להדליק נ"ח תמיד לפני השקיעה, ונכון להדליק בבין השמשות
[One who] became a bar mitzva on Chanuka may light the candles during the day [if he is among] those who always light before sunset, but it is correct to light between sunset and nightfall.
In a footnote he refers to a more detailed explanation of the topic (where he presumably gives the reason for all this, taking into account your considerations) in נטעי גבריאל: חנוכה פרק ג' ס"כ.
Reb Moshe Sternbuch in his Shaalos U'Tshuvos says that he does not see a problem with lighting at the earliest Zeman, however if the Minhag of that family is to always light after Tzeis then the boy should wait for Tzeis.