Although any recently upgraded captain can be competant, safe, experienced and proven, my life experience suggests to me that there are degrees of all of these qualities.
If you are on a path of constant improvement (learning=a change in behavior) you will need many more takeoffs, flights, and landings in order for these improvements to occur. In order to have this increase of experience, time will have to pass in your life, i.e.: age.
Can a young captain be better at his job than someone older? I'm sure there are many examples of that, when the older captain has less experinece (time) as a captain, or if he has no desire to improve.
Certainly, an older person has had opportunities for life experience that will exceed those seen by a young captain. The standards the young captain has met are a baseline, not the be-all, end-all measure of the job, but a beginning of a long experienced air transport professional.
Like Caveman, my job is to make the other guy's job easy, whether it is our newest captain, with whom I will fly this afternoon, or one of the owners, who is a seasoned captain and check airman with a 121 background. I'm still older than everyone here, and I probably will always crew with someone younger than myself. That's fine. My first intent is to be the best FO they ever saw, then I'll set my sights on becoming the best captain I can be.
If you always aim for excellence, the rest will take care of itself.