Mark's birthday is in canon. It happened in issue #4, which was in April 2004, so I set his birthday then. When he turned 18, he had friends over, watched zombie movies, and ate pizza. Nolan and Debbie stayed out of the way until the guys went home; Debbie doesn't like those movies and Nolan didn't want to hover. He and Mark watched "Day of the Dead" to round out the day. Most of his birthdays are like this; a day off to just sit and watch his favorite movies with friends.