Considering how the complexity of science is growing with each passing year, it would make sense for people to start working on their degree as soon as possible. However the academic system in place in most countries doesn't seem to accommodate for that, as even the talented children still graduate after the age of 18.
Why isn't there a push to lower the age of people entering college and is it a rational decision on behalf of the government?
In most parts of the US, going to college requires being able to do other above-the-age-of-majority things like signing leases, handling major transport, etc. There's no to little support base for underage students at most universities. In large university towns like Austin, Texas, USA, where I live, there is a program in place for advanced high school students to go to the local community college (ACC) in place of some of their high school courses to get a leg up on attending the University of Texas at Austin. This program has been in place for decades and works well. Students may still live at home and attend both their regular school and ACC while keeping their support base intact.
I attended an early entrance program that served as my last two years of high school and my first two years of university. These programs aren't common in the US, but there are several.