It doesn't really matter what the arguments are. What matters is whether they are valid or not.
Clearly there is a subjective choice about whether a cohort is qualified to make these sorts of decisions.
Or should we give 14 year olds the vote, or 12 year olds ?
I think most people would agree with the concept of a cut off, that at some point people attain the minimum necessary knowledge, life experience and maturity to make decisions independently of anyone else.
It seems strange to me though that this cut off is not a universal point. At the moment it seems to be (mostly) at the age of 18. But now it appears we are going to make some things like voting earlier, and there doesn't appear to me to be much logic or consistency in that.
Why should people not be allowed to drive, gamble, smoke or drink, look at porn on their own in a pub if they are 16 as well ? There are answers to these questions. But this is more about a joined up policy on the age of adulthood and when children should gain adult responsibility, rather than a quick gerrymander in order to get more votes.
The interesting thing is I see there is not much evidence based discussion on why 16 year olds should be allowed to vote. Most of it seems to be centred around "it's their future and they should have a say in it". Very little seems to be around whether 16 year olds are psychologically ready for it, what studies have been performed to demonstrate this and scientifically support the change of process.
It also seems pretty telling to me that most of the people who have experience of 16 year olds appear to concur that they aren't really ready or mature enough to have the vote.
The whole thing doesn't exactly appear well thought through to me. Which in itself is a sign that it is all about a quick gerrymander and less about a rational discussion.