"Converts" is the plural of the noun "convert" which describes a person who has "converted" or "changed from one belief or lifestyle to another."
Therefore, "converts" in this sentence, likely describes people who, even though they may not necessarily be "youthful or hip," they "strap on apple gear" or us other things that the "youthful and hip" might use to "convert" and become a part of the "youthful and hip" group of people.
I hope this makes sense. Feel free to ask any other questions you may have.