My son did not get a phone until he was in Bais Medrash. And as far as “teaching proper boundaries”, believe me, SJS (and I think 90% of the CR will back me on this one) a bochurs judgement at 2:00am is not the same as it is at 2:00pm (and the AM hours are when many teens are up and about).
Putting a phone in a teens hands is like handing a person with a temper a pistol. True, not all temper-given people commit murder, and not all calm people will always walk away from a tense situation, but if I as a parent can help my son make the decision to stay above water, I think I should.
Just like, as a good parent, I should encourage good study habits, (and make sure the environnment he is in is one conducive to good study habits) I should also see to it that, to the extent possible, his ruchnius environment is just as conducive to success