Rereading OP, I highly doubt they were singled out due to age. Though it is likely that they were singled out due to the fact that they were a party of 3 - much easier to deal with one party of 3 if they needed to bump 3 passengers, than to deal with 3 parties of 1, or even a party of 2 and another single passenger. Oldest of the 3 being 17, might not have been so obvious to be a minor, so error might have not been caught due to appearance. Doesn't excuse the airline, but could be a bit of an explanation.