has it been said for sure it's still Timothy on the 2040 League? Aquaman and Aquawoman swapped, he could've passed it to Luke or Duke between Next Batman and Future State: Justice League.
I don't think Timothy is meant to be younger than Luke, who's supposed to be in his early 20s. I've been working under the assumption he's gonna be older Tamara or just a bit younger, so mid to late 20s. so if he's still on the team by 2040 he's likely gonna be in his 40s, not much older than Bruce and Clark should be currently, so he could just be the elder statesman of the team.