They finally figure it out when Jason is 27. When Selina of all people comments on how he looks like he hasn't aged a day (before kicking him off a fire escape and escaping with priceless gems).

Jason then stands in front of the mirror and looks at himself. Really looks at himself.

Remembers still getting carded when buying beer. How the cashier he was fliting with assumed he'd just started college.

He still looked 18.

It takes Bruce less than three hours to confirm it.

He's not aging.

X

He tells Dick while he's training on the uneven bars. They've discovered he tends to take bad news better, fly into less rages, when he's moving.

At first, Dick laughs. Then realizes Jason is serious. Then laughs again, and continues flying.

Jason always thought it'd be Dick who'd live forever. Peter Pan. The boy who never grew up. The man who's forever young. The pure and perfect Dick Grayson.

Dick flings himself from the bars, twists in the air, and lance gracefully. He gives a mock bow.