I've always assumed that the government believes that we don't need the tax break as much at this point due to the fact that at 16, our children are old enough to get a job that will contribute to their support.
My oldest is now 18 and I know our expenses are much less than they were when she was 13, 14, 15. Her weekend job allows her to purchase her own clothes, make-up, gifts for friends & family, etc... She's been able to save for her own car and now pays for her own car insurance policy, gas, car maintenance & repair, etc... Since she's always divided her money into thirds beginning with the first dollar she made (1/3 towards Spending, 1/3 towards Car, and 1/3 towards College), she already has several thousand dollars to be used towards her college living expenses this Fall - thankfully, scholarships will take care of tuition, books, and room & board. We are still paying for things like her high school fees & supplies, school lunches, shampoo/cleanser/razors/meds, haircuts... and we currently have a few additional expenses: senior portraits, graduation expenses, etc... We basically cover anything that we consider essential and DD pays for all the 'extras'. I'm quite certain that our yearly cost in raising her is substantially less than when she was younger.