it depends on where the concert is, usually when you're buying tickets online it should say all ages or over eighteens.

buying tickets for a concert then re-selling them for a ridiculous price. only greedy a**holes do this.

This depends. If you want to remember the night then yes, because the shirt can serve as a reminder of the night.

It is much more convenient to buy online. Buying tickets at the door is usually more expensive. You also risk the show being sold out and not being able to get a ticket. The benefit of buying at the box office is avoiding convenience charges from ticket selling websites.