Alabama wiki listingAlabama has conflicting language depending on which department you are asking. Safe bet...if you put a "motor" on a bicycle and you want to ride it on the road, you need to get it titled, registered and get a class M driver's license. It appears that a 14 year-old can get a class M license. To get the title, you are going to have to supply either a certificate of origin or some other proof of ownership.
Contact your County license plate issuing office for exact procedures.