A) Mohler is not a pastor, and B) Because Mohler isn't a pastor, he obviously has no experiencing pastoring singles, especially older singles, struggling with their singleness.
A) I'm not sure that's ever a requirement to offer advice. His position at Southern Seminary is one of authority over many young people. This affords many opportunities to offer sound advice to many singles. I'd be more upset if my seminary president didn't offer his feelings on these matters.
B) I would be willing to wager that Dr. Mohler has spoken with plenty of young people during his life. He has also, more than likely, spoken with youth pastors and church pastors as well. I couldn't imagine that he gave terrible advice.
I would personally prefer a leader who says something out of true compassion than one who says nothing from fear of offending. Todays young people are becoming more liberal and worldly and perhaps Dr. Mohler just stepped on somebody's toes a bit?