OK, I'll probably get a lot of flames especially from the younger ones, but hey, I've been there as many of us older ones have and I just wanted to add some things to consider. OK -here goes.
First the good news
assuming you have 5 horses you work with for others and you charge $250/month. that is a total of $1250 for the month and over one year would be $15,000.
Now the less than good news
Your horses (say you have three)need the following:
Vaccines approx $17.00/horse = $51 (assuming you give them yourself)
Coggins $20/horse = $60
Hay at $3.00/bale we'll assume you can get 1 week out of a bale and only have to feed during the winter which here would last close to 5 months from start of needing to feed until grass comes on. That would be approx 22 weeks at $3.00=$66.00
Wormer approx 3 tubes if you use almost exact weight doses (I like to use slightly more)=around $30
Unexpected vet bill (have to figure that in just in case) =minimum $200
Since you will be doing this on your own, you will either be paying rent for a barn or should be paying your parents at least a small amount for using theirs as a business. let's say the landlords or parents are generous and you get off with only paying $150/mo. which would be all but impossible to find around here or I'd be renting a bigger barn LOL = $1800
AMHR/AMHA fees $45+$75=$120
Total horse related fees (not to mention things like trailer sticker fees, insurance, new tires for the trailer, fencing, bedding, etc. etc.)=$2327
Now your cost of living.
Groceries/ household supplies per month without family just you. Let's say your a light eater/drinker and that means maybe you won't go through as much toilet paper either so we'll only figure $100/mo.=$1200
Rent, again we'll figure cheaply at $250 per month= $ $3000
Insurance on your car (don't even know if you can get insurance this cheap for a young driver but let's say $100/mo=$1200
Gas for your vehicle $30/week if prices don't get any higher and you have an economy car =$1560
Utilities for your home-we'll go very cheap again and average out the high cost of heating/cooling and say$100/mo. =$1200
You just developed an abcessed tooth. Need removal and antibiotic. Approx. $150
Now you have strep throat. Dr bill, strep tes,t antibiotics. $200
Neighbors kids are selling junk er I mean stuff for a fundraiser, your neighbor has been good to you so you feel obligated to buy. there goes $25
OK, this is getting long so let's just figure up what the year cost you and what you made for income.
Income was $15000
Expenses were $10662 (and that was figured very cheaply with a lot of things left out that would occur in a year)
Total you made for that year was $4338 divide that by 12 months and you made $361/mo or $90/week or $12/day or $1.61/hour (8 hour day). That's a lot of hard work for so little. I am not trying to say don't do this, but it is impossible to know what expenses will arise in a year and with being self employed you will have to buy your own insurance (not figured in your expenses) and it is not cheap nor does it cover everything. Most any surgery will run over $10000 with most being over $20,000 and insurance will only pay part of that most times. I would just rather see you have an outside job and do horses on the side or go to school to get a degree and work for a large farm where they might offer you housing and benefits. If making a good living with horses was so easy many more people would be doing it. And the people who look like they are making it big....most of them started with a large amount of money in the first place. I hope you don't feel I am trying to put you down or crush your dreams, but better to think about it and work towards it slowly than to jump in ,get way over your head and be afraid to ever try again.
Sorry all that this was sooooo long, but just wanted to show how life outside of the nest was.