In His discourse of 9 September 60 (Chapter 31 of Sathya Sai Speaks volume 1), Swami said,
I will be in this mortal human form for 59 years more and I shall certainly achieve the purpose of this avatar; do not doubt it. I will take My own time to carry out My Plan so far as you are concerned. I cannot hurry because you are hurrying.
I may sometimes wait until I can achieve ten things at one stroke; just as an engine is not used to haul one coach, but awaits until sufficient haulage in proportion to its capacity is ready. But My Word will never fail; it must happen as I will.
That would mean that Swami would give up His body at the age of 93 or 94, since He was born on 23 November 1926.
But He gave up His physical form on 24 April 2011 (Easter Day) at 7:40AM, at the age of 84.
There have been several instances of statements by Swami that appear to be false but, later on, with the right interpretation, ring true. This may be one of them.
Swami sometimes talks in lunar years, not solar years—see the incident below, related by Ghandikota Aryama. Here is a quick calculation, using information found on the internet. Swami lived 30,833 days. Lunar months average 27.21 days, and there are 12 lunar months in a lunar year. Thus, Swami lived about 1133 lunar months, or 94.4 lunar years. That fits with his statement on 9 September 1960.
Aryama's grandfather was the late B. Sri Ghandikota Subrahmanya Sastry, a famous Sanskrit pandit and devotee of Swami. When the grandfather gained merger with Swami on the 1st day of Dasara, 1986, at the age of 93, Swami came to their flat in Prasanthi Nilayam and stated that the grandfather was a maharishi and had reached the age of 100 years (even though he was 93).
After Swami left, Aryama asked his father, B. Sri Ghandikota Subba Rao, about the discrepancy. B. Sri Ghandikota Subba Rao said that Swami uses the traditional Indian lunar years, based on lunar months not adjusted for the solar years, in determining a person's lifespan. 12 lunar months make 1 lunar year. 100 such lunar years equaled 93 years for Aryama's grandfather.