16 Air Assault is the name of the Brigade. A good number of units come under this umbrella, and the chances are that during your time in the Army you will deploy with the Brigade, dependent upon the cap-badge you chose. If you are specifically interested in the Paras you can do this as well. But in answer to your question, yes you can start the enlistment process at 15 years and 7 months. Runtime for the Paras i.e 1.5 miles needs to be done in under 9mins and 15 seconds.
Speak to your army careers office.