Q. A Bar Mitzva bochur who said kidush levana before he became Bar Mitzva, does he have to repeat after if it still is before the end of the zman of kidush levana?

A. Tshuvos R”A Eiger (O.H. 186), Turei Even (Rosh Hashono 27) and Meshiv Dovor (O.H. 1: 18) debate if mitzvos complied with by a minor, have to be repeated when he becomes bar mitzva. However, in regards to kidush levana which is rabbinical Poskim mainly disagree whether a soon to be bar mitzva bochur should wait to recite kidush levana until he becomes biblically liable to keep mitzvos or he may from the onset recite with everyone before. Halichos Shlomo (P’sokim 15: 11) argues that since kidush levana is to begin with only a rabbinical mitzva, there is no point in waiting. However, Yabia Omer (3: 27: 6) maintains that on the onset, when possible (as when weather forecast are favourable) he should wait until the bar mitzva day. He reasons, that a mitzva composed from one single rabbinical level obligation, preempts the one comprised of two or more. Nitey Gavriel (Bar Mitzva 23: 8) suggest that one should wait when possible since the one who complies when obligated is greater than the one who does so voluntarily.
Horav Shlomo Miller’s Shlit’a opinion is similar to the Halichos Shlomo above, and that seems to be also the common minhag.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a