The philosophy is plain and simple: If you're old enough to die for your country, your country should afford you the same rights as everyone else.
This precedent was created in America during the Vietnam war, when Congress passed the 26th Amendment to allow voting rights for those age 18 or older, primarily for the same reason as listed above.
Although there are technically no restrictions on the states changing this law themselves, the executive branch has historically coerced the states into this policy by withholding Transportation funds unless they comply.
Therefore, the solution is simple: Create an exemption for active service members.