Wow, these posts are very revealing and enlightening! I can definitely relate.
I have a 12-year old daughter going to 13. She approached me and she said she wants to be an unbaptized publisher. Part of me wanted to give a good commendation, but another part is telling me to explain to her not to be to hasty with the decision, especially when school season is just about to begin.
With all the arguments and explanations, I thought we ought to follow the example of our Lord Jesus Christ. He got baptized at an age mature enough to make a very serious commitment on serving his Father. And if age will the main basis, I guess 13-15 would be way too young. Earliest could be when one reaches majority age, which is 18. But if we will use the age of the Son of God as primary reference, then 25 years old or older would be more Christian, close to the Christ's age. Neverthless, maturity is something relative, not dicated or determined soley by age.