At what age do you cease to be old?

My oldest son celebrated his birthday on Sunday and my nephew asked him how old he would be last Friday, twenty-five? My son looked at him and shook his head vigorously no with an almost painful expression on his face. "No!" he said. "Don't make me older than I am!" On Saturday my sister asked him the same question when we were at the movies. Again, he shook his head vigorously and said, "No!" (as if being twenty-five was old) I instantly thought of little kids and how they always want to be older than they are. They will even tell a fib to add years to their age. At some point we try to reverse the trend and make ourselves younger. At what age do we start to do that and why?