Not worth it.
1 - You can't compete with bestbuy/dell/whatever name PC's. There prices are too low, and 90% of people will go buy one of those and not want someone to build them one. They like it all to come in 1 box.
2. The other 10% are gamers/hard core users and will build their own PC.
3. At 15, it's going to be hard to get up the money first for a tower, or for people to trust you with say $1000 to build it. Either way, you need $1000 to get the parts first, and wait for them to order, so either you have to float the money, and then if they bail on you, you're stuck with a $1000 pc you didn't want. If you don't have the money to put out, you have to ask people to like come to your house, give some kid $1000, wait a week for parts to come in, come back to your house and get it. No going to happen. I can't get people to do that for PC's and I run a business doing computers.
4. Custom PC places like CyberPower and IBuyer are kicking ass in the custom-built-but-dont-want-to-build-yourself market. People don't want a Dell or HP, they want a custom built PC, they order from there. They buy in bulk and get their parts cheaper than you ever could buying 1 at a time through Newegg.
5. It may be a business, depends on your state and city law. Zoning, business licenses, tax filing, etc all vary by area.
6. If someone does want a custom PC built, odd's are they know some 15 year old kid in the family they will trust with the $1000 a lot more. Everyone knows a computer smart person know. It's like 3 degrees away. lol.
Not trying to shatter your dreams, but the time of being someone who builds PC's and makes money off it are long gone. It's like website design. It's not the deep, dark mystery it was before that only a few people could do. Being someone who has been working with PC and my own businesses over the past 15 years, it changes. No one wants to pay $1000 for a website anymore. You can get a free site from ton's of places. No one wants a website anymore, it's all aobut facebook and twitter pages. Same with custom PC's. You can go to Bestbuy and buy a PC and have it home and running in 30mins. Not a week waiting on parts or you can order online and have someone build a custom one or you will know someone who does.