It's controlled by your wireless access point...I'm guessing you have a single radio access point. It will drop to 54 meg mode if it has a G wireless client connected. If no G clients, it will run at N speeds..but as soon as you connect a G client it will drop its speed to G speeds.
If you want to connect both G and N clients to your access point...and you want your N clients to run at N speeds...you need a dual radio access point that will basically run both modes at once.
Or...upgrade your old G wireless clients with N adapters, so that you no longer have any G clients.