That’s not the argument. The argument is: restrict children from voting due to their lack of maturity in decision-making.
We apply this argument in all kinds of cases that are super-uncontroversial so it’s surprising to hear that it makes someone uncomfortable in this particular case.
If lack of maturity predicates voting, then we're inconsistently disenfranchising individuals within classes. That doesn't seem very fair and I don't really buy feasibility as an excuse. "Sorry, but it's not practically feasible to give you the rights you deserve," is beyond the moral pale.
We apply this argument in all kinds of cases that are super-uncontroversial so it’s surprising to hear that it makes someone uncomfortable in this particular case.