Ah, I read it as "worth" or maybe having spent that much in work across ownership, not as a one time cost. The rest I'm sharing just as a picture of life in the states.
Children in the US can begin work at 14, and assuming he either bought or was gifted a low value project car that he began putting money into from the age of 15 when he would have been eligible for his learner's permit, he could have been earning for up to 5 years before his latest job.
It's also not unheard of for children to own vehicles before they're old enough to legally drive them. There are no license or minimum age requirements for ATVs in some states, and in rural areas, some children begin using them to commute (off-road) as soon as they're large enough to operate them. One of our local schools had stables for horses, and another had chargers for golf carts. While helmets weren't legally required, the schools encouraged them.
In 2020, McDonald's paid cashiers just over $11/hr, so if he worked there just 500 hours each year from the ages of 15 to 18 (10 hours a week), he would have been able to earn over $20K before he was 19 without much effort - or earned all of that in a single year after graduating high school and working full time.
If he was a car enthusiast who wanted to work at an auto shop, like NAPA Auto Parts ($15/hr base), he'd have been able to earn $15K in two years working the same part time schedule. He also would have received full state and federal taxes back each year, so it would have been near to gross earnings.
This isn't really uncommon here. Everything costs something, so most people have to work starting from their teens. Insurance, for instance, is expensive for young men, so if he hadn't been paying on his car's current cash value (or at all) to control costs, he may have received nothing on the vehicle after it was totaled, but he wouldn't have owed anything. By 19, he'd already be used to living paycheck to paycheck.
And while there's no limit to what someone can spend on their car, I'm also taking $15K as an embellishment. $8-10K is more normal for moderate aftermarket work, including labor, and some people buy, sell, and trade up often. I'd believe a teenager spending $3-8K on used aftermarket and then telling people it was "worth $15K", estimating it would have cost around that if they'd bought new and had the work done professionally.