Why do you think that if you don't do it now you won't be able to for that long? Realistically that just makes me even more sure you can't afford to keep one - you may not believe it but buying is the cheap part! Livery, shoes or trimmer, vet bills - oh a horse can run up £1000s with very little effort! - will all cost you money on a very regular basis, at least buying is a one off cost. Any horse you buy now may still be with you when you're 30, are you saying you wouldn't be able to afford to keep it that long? Seriously, put into a separate bank account what you'd pay on livery each month & use it as a saving fund for a better horse some time in the future.
VERY LONG- TL;DR AT THE END!
I know I would have to wait that long because if I dont get one soon/now it likely wont be bought and paid for by my parents- which means I’d have to wait until 18, then I’d be in college and most likely unable to afford a horse, and then getting out of college at 22 I’ll be fresh in the job market, likely with a degree in equestrian science. Probably end up working minimum wage and have to work for the next 8+ years to be able to afford to keep a horse. If i can find a place that has 15 acres I can fit a house and support 4-5 horses with room for a few arenas as well- but a 15 acre property is expensive. A quick google search reveals that it could vary from $89,000 to over $1mil. Expensive and extremely unlikely to ever happen. Which means I’d board. Now a horse- as you said, buying is the cheap part. On minimum wage I would get $13,996.40 a year if i worked the US average of 34.4 hours a week. For one person to buy food and other groceries on a “moderate” budget per year, it costs $3,000. Now down to only $10,996.40. Cheapest apartment I could find was a 658 sqft 1bed 1bath apartment for $1,248 a month plus $65 application fee and $350 move-in fee. $14,952 a year. I’m already broke and in the negatives. If I made an AVERAGE salary in the US of $22.6 per hour and worked a 40 hour work week, $905 a week. $50,680 a year. Minus groceries, $47,680. Rent + one-time fees, $32,313. Healthcare, $27,701. Insurance, $20,870. Taxes, $10,381. Clothes, $10,081. Thats all I have left over. If I buy one horse at (what i’ve seen is) an average cost at $2,500, you have $7,581 left to care for it. Board at my stable is $7,020 a year. You have $561 left for farrier, vet, and other expenses such as repairing tack. This is assuming you spent only money on absolute necessities and didnt spend even a SINGLE PENNY elsewhere. Even on an average income a horse is extremely hard to afford.
Assuming a horse costs $2500 to buy, $7020 to board, $1100 for vet to do Dental, vaccines, coggins, one emergency call, and Coffin Joint injections/lameness eval in one year, plus about $1020 for trimming/resetting four shoes. It totals at 11,640 in the first year and 9,140 every year after.
You would need a salary of $24/hour and work 40 hours a week to be able to afford to support yourself and a single horse comfortably. You would have $1521 left over the first year and $4021 yearly from there on out. Considering $24/hour is more than triple minimum wage it’s likely gonna take a while to achieve. Most people dont get a salary like that until 30-40.
In Australia i may be able to get one by 27-35 because their minimum wage is higher and their healthcare doesnt cost an arm and a leg. I want to move there anyways so hey, added perk.
Equestrian science degrees dont really get you that good of pay unless you work with prestigious racehorses, then you may take home as much as 75k+ a year. Working at your friendly NEIGHborhood stable (hehe), you probably wouldn’t make much above minimum wage. However, the US gov. wants to raise minimum wage to $15 by 2024, but tbh that would crank inflation higher and still make it extremely hard to get a horse.
TL;DR: Horses are expensive and you’d need a wage of $24/hour to support them and yourself, which most people don’t achieve until 30-40.