My son is probably the youngest non-Lubavitcher to put Tefillin on a Jew:
He was literally a week after his Bar Mitzvah waiting by the train station for his grandmother who was going to take him to buy a Bar Mitzvah gift.
He came there straight from Yeshiva so he had his Tefillin over his shoulder as most Chareidi boys do.
A non-Frum guy comes off the train and asks if he's putting Tefillin on people and my son said no. The person then asked if he could use his Tefillin which my son answered, "of course". And he did.
The absolutely crazy thing is that when my son told my wife in the morning that he would be meeting his grandmother by the train, my wife had asked him what he would do while he was waiting and my son, knowing that he would have his Tefillin with him, jokingly said, "I'll put Tefillin on people".