Just to clarify, you are senior to everyone older than you hired seven days later. Who cares if your number 20 or 30 in class when you have 4000 junior to you in 2 years. Every class is random, roughly half retired military, half regional. You may be on average, someone 25 years old will be in the bottom 10 percent seniority for their class. So being that junior in class you will probably get the 756/777/787 in EWR. You have to be senior to hold 737 or 320 in class. Probably the average age other legacies hire is within a year or two of UA.