Generation 1 Clones were created 10 years before Episode 2, meaning that the first generation were 20 at that time, and 26-27 by Ep. 3. Rex in gen. 1, and he looked similar in age to the Batch. However, with the Batch themselves, it gets confusing. Crosshair looks far older than the others. We also know that Crosshair's number was CT-9904, making him the last of the original Batch to be born. You would then think that he should be younger-looking.

We also know, however, that many of the Batch's mutations have side effects. Wrecker is stronger but is less mature and intelligent, Tech is far more intelligent but has poor social skills, and Crosshair has improved sight but a very abrasive personality, and possibly faster aging. Hunter seems to have no side-effects (that we know of). If Omega aged twice as slowly and was created simultaneously with the other clones, she would have been eight during the Bad Batch (which is not outside the Animation Error Range, an error bar that is created when animation makes it harder to judge a character's age of features. This range was very large, for example, with Ahsoka during the Clone Wars film and first few CW seasons).

Now here is where the now-defunct (probably) phase 3 plan comes in. Omega is probably named such because she would be the last clone to die of old age (even later than Boba). What if a mixture of Omega's DNA, along with the DNA of the Batch, would create a super-being with all strengths and no weaknesses.

Omega seems to have what each member of the Batch has not: Maturity and intelligence beyond her age, a natural skill with people and aversion to being alone, and massive amounts of compassion and love. Imagine a super-strong, accurate, hyper-intelligent, tracking, compassionate, mature, and loving clone. A perfect figurehead for any government (in theory--although Palps would have other ideas no doubt), and also the first real effort at making a clone that was more valuable than Jango, not less or equal.