All the other answers seem rather convoluted, so I’m just going to state the obvious: you can get the underlying DatePicker from a DatePickerDialog (by simply calling getDatePicker()) and set its bounds using:
setMinDate(long minDate)setMaxDate(long maxDate)
Where the argument is the usual number of milliseconds since January 1, 1970 00:00:00 in the default time zone. You’ll still have to calculate these values of course, but that should be trivial to do with the Calendar class: just take the current date and add or substract x years.
The more attentive reader will notice that the aforementioned setters weren’t available until API level 11. If you’re targeting that (or newer) platform only, then you’re good to go. If you also want to support i.e. Gingerbread devices (Android 2.3 / API level 9+), you can use a backported version of DatePicker in stead.