There isn't, nor should there be, an age limit. That's the way the tax code works. If someone is DEPENDENT on you to live, they are considered a dependent.
Ok, so if my son was disabled and I had to care for him for life, is he not dependent on me? What if my parents are homeless and I support them by letting them live at my home? Are they not dependent on me?
At some point, the government decided it was a worthwhile endeavor to "reward" people with a tax break if they cared for others.
Your "logic" has another flaw. If my son claims himself as a dependent, he will get the 12k personal credit. I would only get a 500 dollar deduction (don't think adults qualify for the standard 2000 deduction, but I may be incorrect). The taxes he would have paid would have been more than my credit, so it actually costs the taxpayer more.
ALL of my tax dollars for my entire life would not be enough to fund a single redundant, ineffective, worthless, permanent department of a single agency. If that is whining, so be it...
"Of all tyrannies, a tyranny sincerely exercised for the good of its victims may be the most oppressive" -- C.S. Lewis
If the only way to combat "global warming" was to lower taxes, we would never hear of the issue again. - Anonymous
"Society in every state is a blessing, but Government, even in its best state, is but a necessary evil; in its worst state an intolerable one" — Thomas Paine, 𝘊𝘰𝘮𝘮𝘰𝘯 𝘚𝘦𝘯𝘴𝘦 (1776)