Alberto, the twin-paradox actually consists of
two paradoxes.
The first is the one you've stated, and formulated as "How can two twins age differently?". This one can quickly be understood -- it follows from the fact that the speed of light is a constant that has the same value to every observer. If you accept that, all the equations follow and this paradox is nothing but a different "line-length" in spacetime.
The second is trickier to solve, and is the one we're talking about. It is as such: "If all observers are equal, how come one ages MORE than the other"?
Even from your last statement:
The first pilot claims : the time on board of the other ship flow slower than my time
The second pilot claims : the time on board of the other ship flow slower than my time
that paradox is not resolved. Quite the opposite, in fact! The quote implies that BOTH twins still have the same age -- only that they will (somehow) seem younger to the other guy. But that is *NOT* what happens: Twin A is OLDER than Twin B. But how can that be if we've stated that every observer is equal, or everything is symmetrical, as you've stated it?
The reason for that is that in order for the two twins to meet again, Twin B had to turn around, and thus, he "changed inertial systems". Due to that, the problem is very easy to calculate for Twin A (on earth), yet difficult [but not impossible] for Twin B (in spaceship). But hearing "all observers are the same" suggests that the calculation should be the same, though it isn't for the reason given above.
I hope that cleared that up?
