I love those fics where the Justice League first realise Bruce has kids, but I haven't seen a single one where they do the math
Superman: *looks from Batman to his kids, then back again*
Superman: I thought you said you were in your thirties?
Batman: Late thirties, but yes.
Green Lantern: *gestures to Nightwing and Red Hood* These two are like 30!
Nightwing: Rude! I'm 26. I don't look that old, do I B?
Nightwing: I wasn't talking to you, gumball. Besides, at least I'm not 21 being called 30.
Red Hood: At least I didn't have to stay home from patrol last week because I threw my back out, old man
Nightwing: *launches himself at Red-Hood knees-first*
Batman: Boys! Stop fighting! You're both full of youthful vigor
The Flash: *shaking his head* Babies having babies