The brain does keep growing until 25, but the last part to drop in primarily deals with facial recognition. I am dubious about the age of consent being 16 here in the UK, though I understand why it is (kids be having illegal sex with each other a lot).... but I think there should be a mandated age - but also, and I don't know your age, so I'm guessing it's somewhere over 21 - this is an incredibly socially oppressive thing to do to a subsection of society. Would you be prepared to do the same to the over 80s? Your memory is going, your judgement is clouded - you can no longer vote or play a meangingful part in society because we (the middle people) have decided you lack some level of functional capacity.

Where I do think we need to look at things is that there should be a SINGLE age for adulthood. You shouldn't be allowed to do age-restricted things like vote, join the army, drive a car or have sex until ONE set age. The line has to be drawn somewhere and I think 18 is a reasonable stab towards the right place - given how many cars are stolen, drinks are sneaked, movies are watched and if you've ever debated an angry 17 year old on political matters (and I have) then they are just as informed as much older people.. In some instances, I'd rather share my voting rights with well-informed, intelligent 15 year olds than with 40 year olds who haven't really gone past the end of their street and have never read a book.

Here's the Penguin take on age gap relationships if you're interested. https://medium.com/p/22e09ae93b1a