Another quote from AFfC and this one from the mouth of Aemon himself:
"The old man heard him. Though Aemon's eyes had dimmed and gone dark, there was nothing wrong with his ears. "I was not born blind," he reminded them. "When last I passed this way, I saw every rock and tree and whitecap, and watched the grey gulls flying in our wake. I was five-and-thirty and had been a maester of the chain for sixteen years. Egg wanted me to help him rule, but I knew my place was here. He sent me north aboard the Golden Dragon, and insisted that his friend Ser Duncan see me safe to Eastwatch. No recruit had arrived at the Wall with so much pomp since Nymeria sent the Watch six kings in golden fetters. Egg emptied out the dungeons too, so I would not need to say my vows alone. My honor guard, he called them. One was no less a man than Brynden Rivers. Later he was chosen lord commander.""
Aemon therefore went voluntarily to the Wall because Egg (King Aegon V) would not have allowed him to be forced.