Just looking at the trailer for the episode I have to say that, despite his long hair and obvious depression, Future Barry looks good for 36 years old.
Future Barry is still mourning the loss of Iris eight years down the road, and while that sounds reasonable, he looks like a guy who’s still feeling as though it happened yesterday.
That seems to be the challenge that Gustin is referring to as he indicates to TVLine the difficulty in selling the change in character without looking old and haggard ala future Oliver Queen in Legends Of Tomorrow.
So, at first I was about to blast Grant Gustin for not knowing his math and pointing out that the gap between April 25th, 2017, and April 25th, 2024, is a span of seven years, but some quick Google-fu revealed to me that Barry Allen’s birthday is somewhere between January and March 18th, 1989, so he is correct after all.