So after searching high and low I came across the Halachically Speaking (Vol. 13, Issue 2) on the topic of Barmitzvah and Tefillin - see p.7. Rav Moishe Dovid Lebovitz there makes a clear distinction and helps to provide a very comprehensive answer and I have tried to link most the footnotes so everyone can easily access the sources.
He writes as follows:
There is a discussion in the poskim if a bar mitzvah boy recites a Shehecheyanu when placing the tefillin for the first time. On one hand, he should recite a Shehecheyanu since it is an exciting time.1 On the other hand, this brachah was only instituted for a recurring mitzvah.2
If he starts before his bar mitzvah, no brachah of Shehecheyanu is recited.3 Before bar mitzvah he is still a child, so he does not recite Shehecheyanu. When he becomes bar mitzvah, it is no longer his first time.4 If he starts when he becomes thirteen, there is a debate regarding Shehecheyanu. Therefore, he should buy a new garment and make a Shehecheyanu on it, and have the tefillin in mind.5
1 Rambam Hilchos Brachos 10:2. Others say it is not a joy since one has to make sure to have a clean body (Bach, O.C. 22; Sha’arei Teshuvah 22; see Bar Mitzvah [Kroizer] pages 116-117). Refer to Mor U’ketziah 27; Chinah D’chaya 89; Keren L’Dovid, O.C. 57; Chasam Sofer, O.C. 55.
2 Refer to Shach, Y.D. 28:5; Y.D. 289:1; Levush, O.C. 22:1; Taz 1; Elyah Rabbah 2; Aruch Hashulchan 3; Machzik Brachah 2; Kaf Hachaim (Palagi) 10:8; Yufei L’lev 22:1; Me’asef L’chol Hamachanos 19; Birur Halachah 22; Chanoch L’na’ar page 37; Halachah Berurah 22:5:footnote 12; Piskei Teshuvos 22:footnotes 7, 10 in depth; Yechaveh Da’as 2:31; Chayei Moshe 37:page 182; Mishneh Halachos 6:6; Bar Mitzvah [Kroizer] pages 116-120 in depth.
3 Eishel Avraham Butchatch 37; Likutei Maharich 1:page 19b (old); Birur Halachah 22; Natei Gavriel Bar Mitzvah 37:6; Lehoros Nassan 2:9.
4 Eishel Avraham Butchatch 37. See Chanoch L’na’ar 10:footnote 3.
5 Biur Halachah 22, “koneh”; Likutei Maharich 1:page 19b (old), 22:page 31; Yechaveh Da’as 2:31; Mishneh Halachos 6:6; Bar Mitzvah [Kroizer] pages 123-124; see Yalkut Yosef 37:7. In regard to reciting a Shehecheyanu on tefillin of Rabbeinu Tam which one puts on for the first time, see Bar Mitzvah [Kroizer] page 124:footnote 18.