I say that all adults should be able to compete as unattached runners in HS meets. The only caveats are that they should be charged a steep entry fee, wouldn't be eligible for any awards, and can't participate in the post-season championship races. The results sheet should just have their times and "N/A" for their place.
Why? Because opportunities for adults to compete are nearly non-existent aside from marathons and road 5Ks. When was the last time your local park or golf course had an XC race for adults? When was the last time your city closed down a few blocks to hold an 800m street race? And if you're an adult sprinter or even an 800/1500 specialist, how do you get to compete regularly?
I see this as a win-win for both sides. Kids get more funding for their athletic programs, while adults get more racing opportunities.