The twin paradox has been analyzed many times, and many answers are given. Dr. Parker has pointed to the multiplicity of the answers as proof of its underlying error.

The standard answer brings in a new set of rules. While the paradox is posited in the frameword of Special Relativity, the solution uses concepts in General Relativity. "The stay at home twin experiences no acceleration, and ages faster than the travelling twin, who experiences accelerations". That was also how Einstein squirmed through the solution. Not much of an answer if you ask me. But this is the answer taught in physics schools, from MIT to CIT. Is it correct? Of course not! Can I prove it? Of course!

Thus I posit the David Twin Paradox:

Twins A and B leave two planets and travel at equal constant velocity towards each other at total relative velocity v. Twin A sees B moving towards him and aging slower, and vice versa. When they finally meet, who is younger?

Relativity analyses tells us both are younger. Since the accelerations are equal, if any, then these cancel out. Still, one sees the other as aging slower!

The common sense answer is that both age normally and are equal when they meet.

This is probably the simplest proof of the failing of Relativity.