For now, if the Mongoose fits and works then keep it and keep saving. Look at your leisure for a solid quality used bike. My current main ride was a freebie. It came from a friend of a friend. A Specialized Hard Rock that I added fenders, a rack and lights to. When the knobby tires wore out, I replaced them with street slicks.

You may also want to get friendly with the people at a local bike coop. You can learn about mechanics and maintenance, and if a good bike comes their way they will likely channel it in your direction. Estate sales can also be good sources for decent bikes at good prices.

At 16 the only thing that is certain is that your needs are going to change a lot in the next few years. Whether you go to college or enter the work force or the military life won't be the same and neither will your bicycle needs. So you may want to postpone a bike purchase unless you need one now.