The plot of My Growing Pains involves the doctors throwing a surprise party for a colleagues 65th birthday that leads to the discovery of his age and his retirement.
They find out that he is in fact 65, and throw him a surprise birthday party. However, the reason that Kelso never mentioned his real age was because he knew that when he turned 65, he would have to step down as chief of medicine. Kelso reluctantly agrees, asking only that the fact that he was retiring be kept a secret. Sure enough, a board member later approaches Kelso in his office and informs Kelso that during the upcoming months they would be searching for his replacement. 
