Is 28 too old for the tests? I'm 28 and looking to go to college for real this time (looking to pursue my dreams and whatnot). I took the ACT at 18, and didn't take it seriously because I was "too cool for school" and I now regret it and want to go back and restart my last 10 years of messing around over. Is there any kind of age limit that's frowned upon?
No, age 28 is not too old for either of the tests. For the ACT, you should be fine just registering online as usual; for the SAT, however, there have been some policy changes that mean you'll have to jump through a few extra hoops in order to successfully take the test.
The College Board has recently clarified its stance on non-students (many of whom work for test prep companies) taking the SAT: now, people "not taking the SAT for one of its intended purposes" can only register for dates where the full test is released (usually the May, October, and January tests; this may change as the College Board phases out the January SAT test date).
If you do register for the SAT on one of the other test dates, you may receive a survey from College Board asking you to confirm that you are taking the test for its intended purpose (i.e. to apply to college or scholarships). Since you would be doing this, you should be able to simply answer "yes, I am taking this test to apply to college" and be done with it; the reality this, however, that College Board might decide you're too big of a "security risk" (because older test-takers might be planning on memorizing test questions and sharing them illegally) and you might have to call Customer Service to explain the situation.
Based purely on hassle of registering as a 21+ person, I'd say taking the ACT as a 28-year-old is probably easier than taking the SAT as a 28-year-old; however, both tests do currently allow non-high-school students to take the test (particularly if the students are taking the test in order to apply to college).