Well, I'm not an expert as I don't have a job at either, but my best educated guess is that it ultimately depends on the number of flight hours you have. The minimum age at regionals is usually 21. At the majors the minimum age is 23, and most prefer that you have a four year degree before they'll even consider you.
So, on average, I'd say the youngest people hired at a regional are between 22 and 24, and most people are hired at a major between 25 and 27.
Hell my mom got hired at 19 as a FO/FA for some English carrier flying EMB110's. So I guess you were never to young then. You didn't even need a comm. or M/E rating, all the insurence people cared about was having a PPL in the airplane incase somthing happend to the pilot. And no it wasn't PFT, she got paid.
Just think about all those young pilots in WWII. 18 and 19 year olds flying fighters and bombers in the war. That is amazing to think about. These guys were just out of high school and were out changing the world forever. Now, when you the public sees a young regional pilot they ask if they're old enough to fly planes. Crazy how times change.
You also have to consider what kind of connections a person has. If you did an internship or something along those lines, you have a better chance of getting hired with the airline you did the internship with, plus they lower their time requirements too.