Who may write tefillin?

A child is established as a bar mitzvah through shonim (years) and s’aros (hairs)—specifically, he must have reached his thirteenth birthday and grown two pubic hairs. Until he reaches both milestones, he is halachically categorized as a child. Once he turns thirty-six years old, he is considered an adult even without the hairs. Tefillin must be written by an adult male. Therefore, only a person who has met both criteria of manhood can write the parshios of tefillin.

Who may be part of a minyan?

Only an adult male can count towards a minyan. Although there are generally two criteria necessary to establish adulthood in a male, with respect to a minyan it can be assumed that a thirteen-year-old has met both criteria. A minyan is only a mitzvah mid’rabonon. For a mitzvah mid’oraisa, it is not enough to assume that the thirteen-year-old has met both criteria. This is why there must be a clear knowledge that the sofer is a halachic adult in the case of writing tefillin.

Which has precedence: tefillin or mezuzah?

A person who can afford either tefillin or mezuzos should purchase tefillin, since they have greater kedusha. Furthermore, tefillin is a mitzvah performed on a person’s body, whereas mezuzah is only affixed to his doorway. If it is possible to borrow tefillin, the money should be used to purchase a mezuzah. If one is not sure if he will be able to borrow tefillin, the poskim disagree about whether he should still purchase the mezuzah instead.