Lines to enter casinos (and waits for places (seats) at tables and machines) are a phenomenon found in some states that heavily regulate gambling facilities (restricted hours, maximum number of players admitted per hour, etc). These things are not found in the wide-open, 24-hour Nevada gaming environment.
Generally: There is no ID check to enter the casino proper. When you sit down to play, if you appear especially young, you may be asked to provide identification, but the process will be completed quickly - the intent is to keep the games moving, not to create hassles for customers.