If you're measuring the angle at the back of your knee, it should be between 145 an 155 degrees at the bottom of your pedal stroke. Your saddle is too low. This alone will slow you down. It also sounds like it's about as high as it can go, - meaning you need a different bike. ;-)

Having a saddle that's too low not only slows you down, it's tough on the knees. Youth might keep you from feeling it now, but you may be doing some damage none the less.

It's also hard to get a good measurement so if we were to assume for the moment that the saddle height is OK, there are some other things that you can do.

- replace the tires
- replace the suspension fork with a rigid one
- bar ends

All are things that others have suggested. Before changing the gearing you might want to make sure you really need to. When you say you exceed that maximum gear, what do you mean? When you're in the highest gear does your cadence (number of times you spin the pedals per minute) exceed 100? If not, then the gearing is fine for now.

A good cadence for most people is the 80 to 90 range. A lot of folks when starting out ride at a much lower cadence and in too high a gear. If that's the case for you, work on keeping your cadence up and eventually you'll find that you're covering more ground in less time.

Mountain bikes are geared lower than road bikes but they usually have enough at the top to get you moving at a pretty good clip on the flats. It's going downhill where they may not have enough.