I became a paramedic at the age of 20 (three months shy of being 21). I don't feel that you can set a true age limit on either of the issues at hand. Every single person is different, and I think that maybe we should have an evaluation program before ANYONE is allowed into this field at all.
Most police departments require an applicant to undergo a psychological evaluation to see if they are even capable of dealing with the job at hand. So why not adopt this to EMS, there are fire departments that are heading that way. (I'm talking more beyond the polygraph test and typical questions asked.)
As someone who became a paramedic at a young age, I was discriminated against. I feel that this has to do with a combination of my generation being so mentally undeveloped and the things learned from the older generation who developed EMS.
I have had issues with paramedics accepting me, but once they saw that I was serious about doing this, that I wanted to learn, and that I did not have that "I know it all" attitude, they relaxed. I now have four to five crews that work under me, a mix of paramedics and basics, and all have faith in me.
So to end this statement, it depends on the person in question and I feel that generalization of an age could be bad for EMS. I also agree with Dustdevil, that 20 seems about right seeing as how most states require the age of 18 to become an EMT and most decent paramedic programs take about 2 years to complete.