I believe age 18 came about because of the Vietnam War draft. Old enough to die; old enough to vote & drink. Something to consider.
The tax system is a problem also. There's an agency issue; it all about getting someone else to pay more taxes. Don't want to open that can of worms now.
Landownership, by proxy, also used to mean "white male". I'm 100% opposed to race-based and sex-based litmus tests like that, but I:
A. Remain unconvinced that, conceptually, the landownership aspect by itself (in a vacuum, devoid of the race and sex overtones), was a bad thing - land owners have a vested interest in the health of the political system that influences the value of their land in a way that renters simply do not. This isn't a diss against renters (I am one), and the reasons so many people are relegated to renting are a long list of problems of their own.
B. Firmly believe we can design such vested-interest litmus tests that do not facilitate discrimination of any protected classes.
I also firmly believe 18 year olds shouldn't be dying in pointless imperial crusades, regardless of whether they're in Vietnam, Iraq, Afghanistan, or Ukraine. Most 18 year olds of today (at least in the US) are very much so still kids in many ways. No more dead kids.
Your consideration suggestion is noted but slices both ways. Not old enough to drink or own a handgun? Not old enough to drive or die in pointless wars.
Well somewhere along the line we changed our minds on the drinking age, which is 21 in all 50 states