R’ Chaim Kanievsky, in his sefer on Maseches Mezuzah, wants to say that 3.5 is “lav davka” but brings two proofs otherwise and seems to conclude that it is specifically 3.5 years.
The source is Yoma 11a.
As far as tefillin, if worn regularly, it is recommended but not required (practically, sofrim may recommend against it, as shlomozalman said). If worn inconsistently, it is a requirement. If they fell into water, it seems that they need to be checked immediately.