when is your wife 21 ? if soon the embassy may sit on your visa app until shes 21, or issue the visa (as by the time you get it and enter the uk she would probably be 21 or near to) but if its many months they might refuse it
..
really need to know when shes is 21, but someone on here got a visa a few months b4 they were 21, could well be at the discretion of the case worker but the law is you need to be 21.
i think a visit visa can only be extended in exceptional circumstances and i doubt your wife would qualify for that.