Teenagers and young adults are often rebellious and don't want to do what their parents tell them to.

I'm basing this mainly on fifteen-year-old memories of myself as a teenager, but it seems to me that even teenagers who are academically rebellious in general may be more receptive to services that cast them as exceptional, especially if they can be spun as an end-run around what they (correctly) see as an ossified and intellectually sterile school system. I agree that you mainly want to be appealing to parents, though; there simply aren't enough teenagers that are smart enough to benefit from targeted education and independent-minded enough to pursue it and have enough money or pull to fund it.