I don't think the age groups for "MBTI actualization" are set in stone, they're dependent on your environment and family/cultural experiences.
IOW, if you are in situations that render your dependable functions earlier than those stages, then you will develop functions earlier and maybe out of sequence even if you prefer other ones more.
Yep, they are typical age ranges. If you work hard you can get there early with lots of hard work. Some people actively deny these changes and remain perpetually stuck in their past self.
People with tough circumstance many times will remain in survival mode.
It's not to say that people are dumber or smarter than others. They just spend their time developing one part of their brain rather than expanding into the unknown dark areas of the brain.
The hundredth monkey effect is a prime example of this self limiting behavior. Ideas reach critical mass before they become acceptable. If for whatever reason (nature or nurture) your brain vessel (Jung) does not reach critical mass, then you can not ascend into the next level.
Or if you're a gamer...
If you only play in low levels for XP, you will never learn to defeat more complicated enemies in higher level areas. You may accumulate massive amounts of XP, and can buy weapons and armor in which you have no skill to wield.
At the same time you can venture out into new areas early and get your ass kicked or learn to play at a more advance level (game-age) before your actual level is reflective of the enemies around you.
Life's a game... A really difficult game that is rarely (if ever) mastered.