At age 24, Nolan Ryan threw 152 innings. At age 25, Ryan threw 284 innings. At age 26, he threw 326, and at 27, 332. He went on to pitch to age 46, never missing a season, while his second lowest innings output was 149 in 1981 when he had a 1.69 e.r.a for the Astros. His final season he only threw 66 innings, while tearing a ligament in his elbow during a game.
It's all hit or miss, luck or bad luck, in the DNA and a guys physical makeup or maybe a flaw in his delivery. Plenty of guys throw tons of innings while never having any problems and others are injury prone. That's just the way it is.
As for signing Prior, it's a no-lose situation. If he starts at Louisville, and has success, he could be a valuable trading chip or add needed depth if the major league rotation suddenly goes down with a rash of injuries.