Unlike with a contract-based service where laws do not allow T-Mobile to put a phone under the name of someone 18 or under, there is no legal reason that you should not be able to get service with them at 17 under that plan.
You *do* realize what the "real" unsubsidized prices of cell phones are, correct? A simple "free" with contract phone may cost you around $275, and a "smart" phone like an Android device will run you upwards of $530 for the Nexus One, and some of the "lesser" Android devices are probably even more...