Aaishah (ra) was 18 or 19 years old when Prophet (Peace blessing be upon him) passed away and Aaishah (ra) died when she was 70 plus. How is that fair with her to lead the life of celibacy for many years?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

The Mother of the Believers, ‘Aa‘ishah was eighteen years old when the Prophet passed away.

ʻUrwah related that the Prophet married ‘Aa’ishah when she was seven years old, and she was handed to him as his bride when she was nine, with her toys and all. And when he died she was eighteen years old. [Muslim]

Allaah The Exalted forbade the wives of the Prophet from remarrying after his death as stated in the verse that reads (what means): {And it is not (conceivable nor lawful) for you to harm the Messenger of Allaah or to marry his wives after him, ever. Indeed, that would be in the sight of Allaah an enormity.} [Quran 33:53] The wisdom behind this decree has been highlighted by the scholars. For instance, Al-Qurtubi wrote in his Tafseer,

“Allaah The Exalted forbade marrying the wives of the Prophet after his death and declared them as Mothers of the believers. This is one of the special privileges granted to the Prophet to preserve his honor and emphasize his high status to the Muslims … . It was also said that the wives of the Prophet were not permitted to remarry after him because they will be his wives in Paradise, as the woman will be the wife of her last husband in Paradise. It was narrated that Huthayfah said to his wife, "If you want to be my wife in Paradise, don't marry anyone after my death. A woman will be with her last husband in Paradise."”

Since this is Allaah's decree, no one can reject it or be critical of it.

Allaah Knows best.