I will do my best to answer your various questions:
Firstly, the Marriage Allowance and the Married Couple's Allowance are two different things.
1. The Married Couple's Allowance is an allowance is available to couples if they meet the marriage and age requirements. The tax relief you receive is 10% of the allowance you are entitled to. In 2015/16 the maximum allowance you could receive was £8355. As such the tax relief (amount deducted from their overall tax bill) they could receive was £835.50.
If your adjusted net income exceeded £27,700 the allowance you were entitled to was reduced by £1 for every £2 over the £27,700, with a minimum allowance available of £3,220 (tax relief at 10% = £322 deducted from overall tax bill).
Depending on when you married depends onwhether the allowance is calculated on the husband's income or the spouse / civil partner with the highest income.
HMRC guidance here https://www.gov.uk/married-couples-allowance/overview .
2. The Marriage Allowance is not age restricted. It does have other restrictions though. These are (a) the spouse / civil partner with the higher income must be a bsic rate taxpayer and (b) the spouse with the lower income must have an income level which means that they have not used their full personal allowance (£10,600 2015/16).
The Marriage Allowance works as follows. The spouse / civil partner with the lower income informs HMRC (online application) that they wish to transfer their used personal allowance to their spouse. Assuming they have not used the personal allowance they can transfer up to 10% (£10,600 x 10% = £1,060). This means the personal allowance for the spouse / civil partner with the lower income is reduced by £1,060 to £9,540. Correspondingly the personal allowance of the spouse / civil partner is increased by £1,060 to £11,660.
As the spouse / civil partner is a basic rate tax payer they will receive tax relief on this extra £1,060 worth of personal allowance at the 20% rate. This means they will pay £212 less on their overall tax bill (£1,060 x 20% = £212).
In terms of the tax calculation, HMRC simply adds the transferred Marriage Allowance to the personal allowance and this is deducted before the amount of income onwhich tax is due is calculated.
There was some confusion in 2015/16, as the HMRC systems did not initially apply the Marriage Allowance. asa result they had to issue revised tax calculations later in the year. This system issue is now resolved.
HMRC guidance here https://www.gov.uk/marriage-allowance.
Hope this helps clarify.
Edited at 27 Feb 2017 05:31 PM GMT