Can't marry til 21?

I came across this article in the Mail (ssh..)

It's about a Canadian who fell in love with and married a British man - she unwittingly overstayed her tourist visa and is being deported, not realising you still need to apply for a marriage visa

Fine, I appreciate the rules that these days you need to apply for a marriage visa - however, why exactly do you need to be 21?

The age of consent is 18 - why does this lady need to wait 18 months to be eligible? The age was apparently raised to prevent forced marriage...ok then...

I would ask two things - 1) would it not be better to increase scrutiny on foreign marriages rather than apply an unfair blanket ban on those under the age of 21? and 2) Is this a breach of human rights? Sounds like age discrimination to me...