I think there are two different issues here. For her, personally, the decision to overstay her visitor's visa in order to get married was bloody stupid and is actually the cause of much of her problems, rather than her age. I can understand the UK Border Agency taking a hard line on overstayers of any visa for any reason, although if you look at the number of deportations that have occurred they rarely enforce visa limits so it is slightly hypocritical for them to take such a hard line in this case.
The wider question is whether the age limit of 21 is 'right'. If he had gone to Canada and they had been married over there, would she have been able to obtain a visa to come and live with her husband in the UK prior to her 21st birthday? The article suggests that this would not be possible, in which case the legislation is bonkers.
The Government's own research suggested the age increase would not be effective in protecting those forced into marriage and would penalise those who enter into marriage voluntarily. The 'trust us, we know better' Labour Government ignored that research (as they've done on numerous other pieces of legislation) and, here we are with a case exactly as predicted. Well, d'uh.
The most disturbing element of this is that, yet again, the Government has introduced flawed legislation going against advice they have commissioned and then refused to do anything to correct it when the predicted problems arise.
Oh, and BigDummy is absolutely correct. based on extensive experience over recent years, if you are doing anything with UK Immigration, take their estimated processing time and allow double from the point of application. Anything else and you'll wind up in trouble.