International Marathon Medical Directors Association did a study showing that there are various effects of running a marathon too early, and they concluded that triathlon competition should stay in the same respects, shorter with age.
The effects of marathon training too early include...
Higher likelyhood of osteoarthritis in the hip
Higher likely hood of foot injuries including plantar fascitis
Less consistency in future competition
Higher likelyhood of burnout
Not too mention Peak Performance throws around the idea that a person may only get 8 good extreme distance races.