Like all Greek generals, he would have fought in the front lines, so as not to incur the contempt of his men. Herodotos says he went down in the midst of the fray, and his surviving troops fought to drag his body away from the front lines before they were overrun.

It was not unusual for Greek commanders to stay in active service well into old age - even when they passed the age of 60 and were no longer liable to military service. Spartan kings were no exception. I wrote more about this here. It was understood that old men would not fight particularly well, but that wasn't terribly important in Greek warfare anyway; willingness to fight was of much greater importance.

We do not know the precise life expectancy of the Classical Greeks. However, even without modern medicine, humans generally seem to be able to reach a respectable age as long as they don't die in their infancy. Low life expectancies are generally caused by high infant mortality; when a child is born, the chance of reaching an advanced age is slim, due to disease, exposure and malnutrition. However, once the child makes it safely to age 5 or so, their life expectancy starts to climb rapidly. Model life tables suggest that a population like that of Classical Greece would have included plenty of men over 60. Anecdotal examples include the Spartan king Agesilaos, who died on campaign when he was 84 years old; the tragedian Sophokles, who died aged 92; and the orator Isokrates, who lived to the age of 98 (meaning he began his writing career during the Peloponnesian War and ended it with a letter to Alexander the Great).