Metric 1: Introduction

Who was introduced first? Since everyone was introduced before Flashpoint, there's only one universe reboot resetting their ages for us to worry about, so this is a fairly safe measuring system.

Bette is first, followed by Lilith, then Gnarrk. Crisis on Infinite Earths happens, and then we get Tara and Joey at about the same time, followed by Danny (don't quote me on that). Wendy is last.

Metric 2: Last Known Age

How old were they when they died or disappeared? General Spoiler Warning for this section, since we're discussing character death. This is mostly for the purpose of keeping the comic book aging time consistent, since comics characters obviously don't age as fast as the real world and frequently don't even age as fast as each other (looking at you Raven).

Bette's last appearances were some scattered guest star features in 2011. Since this is still New Earth Continuity, and she's around the founding Titan's age in New Earth, so I'm choosing to assume she was somewhere in her early to mid 20s here.

Lilith's last appearance pre-Flashpoint was when she got killed in 2003. She was still a main Titan at this time (same age as the fab 5), so also early to mid 20s. Lilith is also the only Joe who hasn't dropped off the face off the planet, and was last seen post-Flashpoint in Power Girl Volume 3, where I think she's the same age as PG, which would put her around 30

Gnarrk's last appearance I THINK was in 2019 in Heroes in Crisis where he got offed by Savitar, but he was part of the OG TT here, which would put him in his mid to late 20s.

Joey's last appearance pre-Flashpoint was in 2004 in Teen Titans where I think he was still alive after being dead for a bit? Anyway, here, he's in the second age bracket of Titans---Beast Boy, Raven, Terra, etc. I wanna say this makes him no older than 20 when he goes M.I.A. He more or less disappeared in Flashpoint, save for a cameo where he isn't a vigilante.

Tara's last appearance was the Judas contract where I think she was about 16 (goodness knows I'll get roasted if I'm wrong). A couple of other people going by Terra appeared after that, but they're not the same person.

Danny was about the same age as Jason Todd and got kicked from the team when Jason died, which would have made him 15. He later got killed/merged with the souls of Azarath into the entity Phantasm, and I'm going to assume through the power of Comic Book Time that he was still 15 here.

Wendy---bless your heart, you're easy---is in the same age bracket as the YJ98 crowd, and was last seen in Batgirl v3 in 2011, where she would have been somewhere from 17-19.

Metric 3: Ages Relative to each other

Some of these people have actually coexisted and interacted, and even if they haven't, most of them have teammates in common.

Bette is the same age as Dick Grayson, which makes her one of the youngest of the OG Titans. Lilith and Gnarrk are closer in age to Arsenal, making them some of the oldest OG Titans. If I had to give it a number, I'd say Bette and Gnarrk have a 3 year age gap at the most.

Terra and Joey are around the same age, and were about 15 when Dick was 19ish, I think, putting a 4 year gap between them and Bette while Danny would have been 12 at the same time.

Wendy is the youngest by this metric, making her the same age as Danny or a year younger.

Metric 4: General Character Vibes

This metric could also be called: how old were they when they had all their important stories? By this measurement we get:

Lilith and Gnarrk: Mid to late 20s

Bette: Early 20s

Joey, Tara, and Wendy: Late Teens (16-19)

Danny: Early Teens (13-15)

Summary: Between all these metrics, a pattern emerges Lilith and Gnarrk being the oldest, followed by Bette. Joey and Tara are older than Danny but younger than Bette. You can either make Wendy somewhere around Joey and Tara's age, or you can make her around Danny's age. For the purposes of this AU, we like to use the first option.

Also. Bonus metric for the sillies:

Metric 5: What Decade are they from