i did this when i was 16 with my birthday money.
i started with $100, which i invested in some video lectures online which taught me how to use html / flash for building web sites.
from this, i built roughly 8 web sites ranging from $500-$1000
with that money, i invested in some molds for carbon fiber products for automobiles. i kept building, and before i knew it had brake, suspension, wheels, and intercooler kits.
at 18 i moved to Los Angeles because i wanted to take all of the research i had done on various car parts, where the best factories were, etc. -- disassemble production cars and put my parts on it [e.g., i wanted to take a bmw 3-series, strip it all down to most of its core, put my body panels on it (so it would look like a different car), my suspension parts (so it would corner better), my turbo kit (so it would be faster), and so on] to fill a small niche market.
however, i did need investors to do this and the general vibe i got from most wealthy people i met was that i was simply too young to be trusted with their money. during this process i sort of fell out of love with the afertmarket car scene, so i stopped making parts as well. i was able to sell most of my sources and molds, though. now i'm getting my degree in industrial engineering.
it's probably for the best, because this was around the time banks were failing, so perhaps the business wouldn't have succeeded anyway due to the market. the depressing thing is that this is what Fisker Automotive was doing around the time i was in LA, now they have a new car (Fisker Karma) which is very cool and what i would've liked to eventually moved the company into. there's a large possibility that the automotbile industry can have a major shift, depending on who comes up with the right technologies.