Are you sure? I assume younger teachers are the ones with younger kids starting school. I don't think paychecks for new starters and teacher assistants are that high. Unions usually care about employees with more years of service to the detriment of new employees.
Starting CPS salary is exactly where the average for the area is, but CPS also gets a huge amount of PTO, a good benefits package, and a defined benefit pension plan, and comp improves reasonably rapidly and (importantly) predictably. Rural teachers have it rough, but most of what you hear about teachers being underpaid doesn't apply so much to major blue city school systems.