This isn't the argument you think it is. Until this year, parental consent was required for 16 & 17 year olds to marry. 16 & 17 year olds were never regarded as legal adults. The new act (
see here) is mostly an attempt to prevent forced marriages before people reach legal adulthood.
All of which is merely academic, because voting isn't marriage. For instance, unlike marriage, when it comes to voting there is no need to protect people from potential harms. Voting is a matter of having enough understanding of the world to apply it. Similarly, the
medical age of consent is 16 or lower - the test is
Gillick competence not legal adulthood.
In short, there is no need 'to bring everything into line' at 18. Different things require different norms.