In Australian and New Zealand Standard AS/NZS 2299.1 Occupational Diving Operations requires that candidates for occupational diver training be a minimum of 18 years of age. This is due to a combination of factors
Bone growth continues in humans until late teens and can continue in individuals into the early twenties. There is some evidence that bone growth can be inhibited or impaired by exposure to the some of the conditions experienced in diving.
Occupational divers are required to be able to undertake reasonably strenuous work including potential involvement in the rescue and recovery of the team mates. This requires a degree of size and strength that is generally directly related to the physical maturity generally occurring in the late teens.
Occupational divers are often required to make decisions based on understanding and judgment that generally comes with age and increased experience.
The minimum age of 18 is the only age specification. There are a number of commercial divers working offshore over the age of 50.
The ability to do this, of course, depends on maintaining your diving medical fitness.
If you want to and you have the skills, you can always move on to supervising or management, neither or which requires a diving medical, but do have there own special requirements.