What I mean is, there is acknowledgement that there is need for *some* number, some limitation. In the case of legal driving age, I think it is safe to say there is strong agreement that 8 year old's should not be licensed to drive cars, trucks, buses, airplanes, etc. So, where to draw the line? Not everyone matures at the same rate, or in the same way, or in some cases ever. It then becomes a matter of compromise, trying to capture as many people who shouldn't be licensed to operate a vehicle against unduly denying those who can be licensed in a reasonable and cost effective way.
In other words, the number that is settled on is not done arbitrarily, as opposed to what you were implying.
Of course, I'm not saying that this process always results in the optimal number being chosen, or that these decisions are always made with all the necessary information and a complete understanding of all the factors involved (like with this wealth cap idea). I'm just saying these lines are drawn with some rationale and not arbitrarily or on a whim.