Whoever does not fast in Ramadan because he has an excuse which is not expected to go away, such as being elderly, has to feed one person for each day. He has the choice of offering this food day by day, or waiting until the month is over and then feeding the same number of poor persons as there were days in the month.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in al-Sharh al-Mumti’, 6/335:
The time of offering the food is his choice. If he wants he can offer the expiation for each day on the day, or if he wants he can delay it until the last day, because that is what Anas (may Allaah be pleased with him) did.