There is not a way to actually shut off the AirPort at a certain time.....unless you add an outboard electronic timer to the AirPort router......but there is a way to define the timeframe each day that a given device will be able to connect to the wireless network.
So, "rules" for Junior's iPad might be to allow access to the wireless network each day from 8:00 AM to 2:00AM
To do that, you actually need to build two "rules".....
One rule to allow access Everyday Between 8:00 AM and 11:59 PM
A second rule to allow access Everyday Between 12:00 AM and 2:00 AM
The crossover from one day to the next is a bit tricky since you cannot set a rule to extend past midnight. You might think that there would be a one minute break between 11:59 PM and 12:00 AM, but the 11:59 PM setting is actually 11:59:59 turning off at midnight. At the same time, the second rule kicks in at midnight and allows access to 2:00 AM.
How 'easy" this might be is open to question. It's easy if you know how or spend some time with the Timed Access settings in AirPort Utility. It's not easy if you are looking to make a few mouse clicks and have everything just happen.
You will also need the MAC Address or WiFi Address of the teenager's device. That is easy to find on a Mac or an iPhone or iPad. Might be more difficult with a 3rd party device.
You can also establish similar "defaults" for the entire network to affect all users......but most parents just want to be able to limit access of their kids' devices, while the parents can connect anytime that they want.