Well, if we talk about class 3B and 4 it is and important question, as these classes should be considered as weapons. Although I think the age is just a number of circles around the sun you already made it is not really relevant. The more important is how mature and responsible the person is. Let's say 18 years? Well, for somebody it might be 16, for somebody 21 or 30, for somebody never.
As I mentioned the lasers of these upper classes considered as weapons it does not mean they should be regulated that strictly as fire arms for example. I believe that in some countries like Australia, UK etc. where the law is very strict on portables, the regulation should be different. Yes, limit access to these lasers for general public in case there are problems with missuse in the particular society - completely understand this. But for laserists allow them to set up clubs or non profit organisations, whose members are certified by passing a laser safety test, going through course and/or something like that. Even for car driving you need a licence. For these members there should be no limit for lasers. However I believe that instead of regulation more information about laser safety and how to treat them would be better, as what is forbidden attracts attention and with lack of information the hazards are significant (it is much easier to shine laser, than to handle fire arm as laser looks so harmless - but it is not). Anyway criminals will always get the weapon (even laser) illegally, therefore regulation affects more those who stay legal, this is not good.
Another reason is that lasers are the future, increasing number of technologies use them and popularity of lasers might bring new young brains to research and developement and boost these technologies. So, excessive regulation does not help that and slowers technical developement.
Just my opinion.