You have to be a minimum age of 18 to get into commercial diving school. Most diving contractors will not hire you unless you have been through one of the recognized schools because commercial diving school graduates are a dime-a-dozen, and it would be a liability to hire someone with no training to start with.
It is my understanding that if you are in good physical condition, you have a good, solid record of experience as a commercial diver that you can continue to work to a ripe old age. I do not think that you will fare well to try and switch careers at age 40 (or possably even 35) to commercial diving if you have no training and experience in the industry. One does not step out of the corporate cubical and instantly become a Journyman Carpenter any easier than leaving a Seven-11 clerk job to walk into an operating room to be a Neurosurgen. You have to pay your dues, have reference to back up you experience and you have to be able to perform and produce a product/service as fast as the next kid. Don't base your expectations of who experienced commercial divers are on those staged "reality TV" shows about divers. Most of the guys they show are either somebodies brother so they got the job, or they are on their way out the door due to their lack of performance and stupidity.
Also, that photo you are using as your avitar will not get you very far in the industry either. Commercial diving is a conservitive, "Heman" industry that is not as affected by the labor and social laws that typical jobs on land are bound by. Drug use, anti-social personalities trates, being sensitive and not keeping your mouth shut will also get you removed from the barge/oil rig/job site in a manner of minutes as well.
Good luck
