The Married Couple's allowance has three levels, just like the Personal allowance. The higher levels are due where either the husband or the wife is 65 or over or 75 or over. The income limit rule continues to apply; that is, only elderly people with modest incomes get the full age increase and there is a sliding scale reduction for those whose incomes are above the limit.
The most important point to grasp here is that the levels of the Personal allowance and the Married Couple's allowance are fixed separately. The level of the husband's Personal allowance depends on his age and his income only. The level of the wife's Personal allowance depends on her age and her income only. But the level of the Married Couple's allowance depends on the age of the older spouse even though the income limit operates only on the husband's income.
So a husband could get the ordinary Personal allowance for the under 65s and the Married Couple's allowance for the over 65s or even the over 75s.

If a man who was under 65 on 5 April 2000 subsequently marries someone who was born before 6 April 1935 he will be entitled to claim the larger Married Couple's allowance appropriate to the age of his new partner.
Similarly if a man born before 6 April 1935 marries someone of any age he will be entitled to claim the larger Married Couple's allowance appropriate to his age.