My dad is 67. He has been told that he has an aneurysm in his knee. Is that common in older people?

Not uncommon. He has popliteal artery aneurysm. Please see vascular surgeon to see if treatment needed now or if it can be followed.
No. Aneurysms are uncommon. Popliteal aneuryms (behind the knee)are the third most common location. They typically do not rupture but rather clot or cause clot and debris inside them to go downstream and block the more distal arteries. These can lead to amputation. It is important to operate on these if > 2 CM in diameter or are causing symptoms.