24/25? Many people are barely out of college at 24, let alone in their internships. In fact, the average age of matriculation into med school is 24, then tack on another 2 years of in-class education. Since the characters have all passed their boards, that's another two years of clinicals that are completed on top of that. The youngest Grey's characters are bordering on 30.
Residencies are 3-5 years. 5 for surgical. Since most surgeons subspecialize, they're going to complete a fellowship. That's another 5 years. Which means the youngest attendings on staff are around 40. Bailey is towards the end of her residency (which is apparent since she is up for Chief Resident) meaning she would be around 35.
And this is assuming all of these characters entered medical school immediately after completing their undergraduate degrees.
"You just can't win. Ever. Look at the bright side, at least you are not stuck in First Wives Club: The Musical. That would really suck. "
--Sueleen Gay