Mark Calaway is 50 before WrestleMania 31 and in constant pain from his history of back problems and severe shoulder issues. The shoulder injury he suffers is in fact so bad that doctors in 2010 decided it wasn't worth surgery. Taker instead had to rehab it and now has no choice to work around it his matches. One thing he did have surgery for was his hip, undergoing operation in 2009. The physical issues are continuing problems, they definitely don't need any further strain.
It's a huge factor in why we could see The Undertaker retiring after WrestleMania 30. From the standpoint of logic, there shouldn't even be a choice, he should be resting up and avoiding further physical aggravation. He's certainly built enough of a legacy to leave behind, there's no question of that. The other aspect of Calaway retiring is he simply doesn't need it any more. He's a millionaire multiple times over and will likely stay on the WWE paid books regardless of retiring. The thought of putting his body through the physical demands of wrestling into his fiftieth decade must be a gruelling one. He could be enjoying the comforts of his family life, instead he finds himself facing one of the most physical performers of all time in Brock Lesnar at WrestleMania 30. Time might be about to get called. Age and physical exhaustion have caught up with The Undertaker.