A stage manager right out of school can make anywhere between $5,000 a year, and $30,000.00 I guess it depends by how quickly you move up the ladder. The number of stage managers out there, working at any given time, a 1,000, 2,000? Who knows, hard to pin down. Most of the jobs are in major metropolitan areas, where indeed the cost of living is high. The job tops about at about $150,000.00 a year.
I am actually siding with your parents on this. If there is anything else that you can do for a living and still be happy (for example do theater as hobby), you are probably going to end up happier in life.
The stats are a little disheartening. Look at AEA's annual report. Less then half of the union members (this granted includes Actors and Stage Managers who work under AEA) less then 50% of the members work, and the average number of weeks they work was a little over 15 weeks. (Granted, actors SKEW this amazing, but interesting facts.) The median yearly salary was just around $7,600.00. Looking at SM contracts, there were 27,878 work weeks . . . which means, as far as AEA work, that means there was full time work for about 536 stage managers in union. (Yes, I know there is a lot of living wage non-union work, non-AEA work, but this is the stats I have in front of me.) 536 full time SMs? (I bet there were about 150 stage managers work full time, about 900 working part time . . . )
Seriously, the odds are stacked against you to make a living wage in this career.
Also, I have to tell you stage managing right after college is going to be the hardest years you have to work . . . you are competing with a lot of stage managers for these entry level jobs. And since you have less experience, you are less marketable for jobs. (It's sort of a catch-22, I don't have enough experience to get living wage jobs, and I can afford to take low paying jobs to get the experience.)
Plus, it helps, especially early in your career, to be mobile, able to move at the drop of a hat to take a job out of town, which makes it hard as well.
Listen, staging managing is a HARD job, especially if you freelance. The job is difficult, it's long hours, tedious (at times) work, little appreciate, low pay often very early in the career cycle (if any pay). As you work your way up the ladder, you will find yourself working 60-80 hours a week, which makes relationships hard, and if you are lucky, and end up working 52 weeks a year, six days a week, 60-hours-plus a week - relationships are nearly impossible and exhaustion and burn out become huge factors.
Your parents are right to discourage you from going into this career. Too many people are entering the world of theater. Think, there about 2,400 year-colleges/universities in the country, if only 10% have a theater program, and they only send out one stage manager a year, that's 240 new people a year trying to get one of those 536 full time work?
I find that too many your people are "the best" of their program, and are built up to think that they can make a living in this business. It's too often the tale - someone is in love with theatre, but can't act, doesn't design, doesn't direct -but has a passion for theater. Someone in the program asks them to stage manage, they find they have a knack for it, and they are offer a service to the department, so soon they are stage managing more shows - and they think they can make a living at it, and the school wants to encourage them (obviously), but at the end of the day - not everyone can make in this business. There are not enough jobs. Too many people don't have the talent, the drive, the management skills, the leadership skills - and end up getting frustrated and spending too much time chasing a dream that is just not going to come true. Balancing day jobs and stage managing is possibly, but you quickly learn that it's very hard to work a day job and be a full-time stage manager.
NOW . . . I know nothing about you, your history, your dreams, your aspirations. But, I am just saying . . . all things being equal, this is a damn hard path.
Are you still in college? Do an internship, get a sense of what the job is like in the real world, see how your skills are marketable.
Sorry, I don't mean to be Debbie Downer, but I don't think your parents are being idiots to discourage you.