There's been a lot of controversy regarding the ages of the members of the Justice League--or the lack of knowledge about them. I'm here with some answers.
In the present day, Wonder Woman is twenty-three years old as stated in Wonder Woman issue 11.
In the present day, Cyborg is 22-23 years old. This one took some figuring out. In Justice League issue 1, college scouts are ready to give Victor football scholarships. Usually, players get football scholarships during their senior year, in which they are 17-18 years old. Add five years to that and Cyborg is between 22-23 years old. There's also the fact that 5 years ago, Sarah Charles (Cyborg's love interest) was noted as being 17. So in the present day, she's 22--which puts her and Cyborg in the same age bracket.
As of now, Green Lantern: Secret Origin is still in continuity. In that story, Hal is 18 years old when he joins the air force. He's kicked out for joy riding his jet. To be a fighter pilot you need a bachelor's degree (which takes about 4 years) and you need to go to flight school (which is about 9 months-1 year). So add all that to Hal's age, and he's about 22-23 years old when he's kicked out of the airforce and becomes Green Lantern. Then add another 5 years to bring Hal's age up to the present day. And we can see that Hal is approximately 27-28 years old.
In Justice League #2, Barry Allen is already a crime scene investigator. It takes about 6 years after graduating high school to become a crime scene investigator (4 years of undergrad and 2 years made up of being in the police academy and doing lower level police work). So add 6 years to 18, and Barry was 24 during the Darkseid Invasion. 5 years later, he's 29. But in Flash #0, Barry is made fun of for being young, so maybe he completed under grad early? If we shorten his undergrad by two years because he's so smart, then Barry is 27 along with Superman and GL. That makes things feel more comfortable universe-eise. So Flash is 27-29.
Aquaman is easy to figure out. In Aquaman issue 8, it is revealed that Arthur lost his father 6 years ago, when he was 18. That pegs Aquaman at 24.
Superman is revealed to just be starting his journalism career during the beginning of Action Comics set 5 years ago. It takes about 4 years to get an undergraduate journalism degree. Some journalists get another 2 years of study for a higher degree. So if Clark is 18 when he finishes High School, then during the Darkseid Invasion, he's 22 or 24. 5 years later, he's 27 or 29. Edit- Superman #14 confirms that Superman is 27.
Batman is the most difficult to figure out due to the Robins. During his still-canon Year One story, he's 25. Damian Wayne is physically 10, but he was grown in a test tube so he could have been conceived within the 5 year timeline. Tim Drake was revealed to be an olympic athlete who was never an official robin, so he probably only got a few months of training from Bruce. Dark Victory (Dick Grayson's origin) takes place 2 years after Year One when Bruce is 27. But here's the thing, during the Darkseid Invasion, Bruce had no Robin. The Invasion couldn't take place during Year One or during Dark Victory, so it probably happened between them when Bruce was 26. That put's Bruce's Age 5 years later at 31. You could say that the D.I. took place after Dick left or Jason died, but Batman is not a pool of sadness like he is after those events. He seems fresh and young. So I'm sticking to the 31 story--at least until we get more information or retconning.
So what we know is:
Green Lantern- 27(-28)
Wonder Woman- 23