Argentine dinner starts at 9 pm for early birds but 10 pm is the regular time for most people to show up at restaurants. Show up at 9:30 pm and you can choose the table you want. Show up at 10 pm and you may be waiting at the door.
Regarding nightlife.... it's going on every night and most places stay open until 5 am and don't even open until 11 pm or midnight. Learning where to go out is an adventure in itself as the best places are often little more than a doorway on a busy street.
So... who's going out like this? People that live the "vampire life" such as tourists, expats, students, night workers and even regular workers. I've known people who stay up all night and then catch the Bus outside a club to go directly to work. They work part-time or half the day and then take siesta for the afternoon.