I've used Twitter Bootstrap for a few designs now, it's a useful tool but requires you to maintain more LESS/CSS than more other frameworks (to support the responsive designs).
If you are not going to support responsive, there are plenty of other CSS frameworks out there to compare against, from Blueprint to the new BootMetro style.
If you are looking to learn web development, starting with bootstrap will throw you into learning LESS (a CSS compiler language) and into the new concept of responsive designs (where the page resizes itself to fit to phones, small devices, tablets and desktops). This can be useful for certain applications, where others responsive designs would be entirely useless.
Overall I've liked Bootstrap, it contains all the usual features you find in a CSS framework, but they throw in some other things most do not, such as a small JavaScript UI library.
Also being 18 doesn't make much of a difference, I was working full time as a web developer since 15 and knew HTML/CSS/etc when I was 9 - you will find many people on Hacker News like that. You should say your confidence level, like "I'm kind of noobish with web development, so forgive any questions if they seem obvious." Replace "web development" with anything.