You're absolutely right. The dragon age categories are a sort of throwback to the times when monsters were not scaleable.
If you wanted a tougher gnoll you invented a new monster, the flind.
In 3.x that is no longer necessary, monsters can (relatively easily) be scaled, given class levels, etc. (Nowadays, if you want a tougher gnoll you make a fiendish half-troll gnoll ranger/dervish....... uh, I digress.)
We never see, say, 5HD fire giant children as the main monster, so why are immature dragons so common?