In SRT literature we can find the "answer" in form of a thumb rule "moved clocks are slower" and using Minkowski interpretation there is another conclusion, that both observers are seing the other as the moved one with slower flowing time. That tehy claim must be symmetrical like it is in length contraction.
But at the same time the established solution of twin paradox tells us, that the moved twin of the two will stay younger. Logically they would be "both younger" then the other one and therefore the relativistic effect would be eliminated.
This old paradox is nit solved, but is ignored by ad hoc rules without explaining a physical reason. Many time people tried to solve that this problem, but no one did found a way. Many did deny relativity at all, but some did try to develop another theory, but failed.
Now we have discovered a failure in SRT really. This is in an inconsequently use of a measurement process. For time attribute we have a time dilation, i.e. enlargement. Lorentz factor inlarges the time in moved inertial frame IF. So then consequently more time must be in the moved IF? But we are told that it must be less time in moved IF. How?
The solution in SRT is, that hidden process of measurement is used, without to understand and explain it. The time dilation only is an interaction on elementary particles on the wave length and frequency and the macroscopic time unit is a multiple of it. Then the relativistically elongated time unit is used to measure how many times it passes during a trip between A and B and consequently they are viewer of elongated time units passing. So we get the result, that viewer time units in moved IF are counted then in stationary one. So for time dilation SRT uses that measurement process.
By length contraction this is not used: all lengths are to be contracted in SRT. Both length units and the measured distances. Just see at contacted space ships and planets in thousands of videos and in SRT literature. The length unit on bord of that space shuttle is of course contacted together with the shuttle?
That leads to eliminating of length contraction and as a consequence even to violation of the constancy of speed of light, the second postulate of SRT. If we contract the length unit and the distance by same Lorentz factor and then we divide the new shorter distance by new shorter length unit, then both Lorentz factors are eliminated and we get same number of measured length units. There is just no measurement process defined in SRT and was used hidden in time dilation nevertheless, so time dilation is the only right relativistic attribute in SRT.
The logical error was installed already by Poincare and Lorentz and was taken over by Mr. Einstein as a truth. Poincare and Lorentz were authority's at that time while Einstein was a new comer.
We have developed a new Relativity Theory, which explains the above twin paradox by Gravitational Level Relativity, which replaces the Einsteins “Relativity Principle” about “equal righted inertial frames”.
To get deeper into thematic we recommend a following lecture
6. Gravitational Doppler and Level Relativity solved a discovered momentum paradox and identified the cause of relativity
And another of 15 articles.
First look at the Moritz-Paradox and that missing measurement process ofthe distances.