I've said this before but the major concerns are James and Jordan.
James' voice obviously won't be the same in 10 years, as it's the only "instrument" that can't be replaced or taken to a repair shop, and 30+ years of touring and belting it out definitely take a toll on someone's voice. Here, they could either retire a lot of songs from the live sets, which is fine considering they have a big enough catalog, or just play the songs in a lower tuning/tonality like the IAW songs in 2017.
Obviously they could always find a younger guy who could nail everything, but that would have to be James' decision to retire, as they seem quite happy with him and wouldn't replace him out of the blue, even if he's been a bit inconsistent in the last few tours.
As for Jordan, I don't think we should worry about his playing abilitiy past 65, just look at Chick Corea, who's still playing flawlessly at age 78. However, there's a chance he might want to retire from the "big" touring life and just focus more on his family, plus doing all kinds of smaller projects. It's all the touring and messy schedules that take a toll on you, more than actually mantaining the chops to play DT material every night (which is quite impressive as it is). More than getting "too old for this", I feel like Jordan would just want a lifestyle change before actually getting too od for anything else.
Given their ages and how long they have been doing this, I would not be surprised to wake up tomorrow and read a press release saying that one of them decided to leave the band/retire due to fatigue or burn out or any other reason. 14 albums and 30 years in, I find it best to enjoy anything we still get.
As I was about to post, this appeared, and I agree, this makes a lot of sense.