Yeah, that doesn't even make sense the way you're using it. She doesn't exist in a bubble where time passes more slowly. Her chronological age adds up the same as everyone else. Her aging at a slower rate, as Meyers said, would mean that if she currently has been alive 18 chronological years, then she should have the body of someone much younger, ala Ms. Martian from the YJ cartoon who was 48 Earth years old, but only had the body of an adolescent. So, assuming Ms. Martian had the body of a 15-year-old when that was said, the rate of aging for her was 3.2 Earth years equaling one Martian year. If we applied that rate of aging to Starfire, then to be chronologically 18, she'd have to have the body of an Earth child about the age of 5 or 6. I somehow doubt that DC would be okay with Dick dipping his toes in those waters.
Obviously, however, Starfire has an adult woman's body. So if she's got an 18-year-old's body in a biological sense, then her actual chronological age would be 57.6 Earth years. So...not really a teenager and therefore obviously "too old" to be a Teen Titan as per your standards. It would also mean that Dick was 17 while she was about 53. So, in that case it goes from Dick being the one committing a crime to Starfire being the creepy one.
Even if the rate of aging was only 1.5 Earth years for every Tamaranian year, then Starfire would still be about 27 Earth years old to be biologically 18. And if she was chronologically 18 with that rate of aging, she'd have to have the body of a 12-year-old. Again, don't think DC would want Dick getting up in that.
Bottom line: Her "aging slower" means that for her to be biologically a teenager, she'd have to be chronologically much older. That would mean she's not really a "teenager" and as such, according to your definition of the "Teen" Titans, she doesn't belong on that team.