The truth is that there was a lot of controversy among the fans when they saw that in the future, Twilight's friends were of legal age, but Twilight has no sign of old age other than having Celestia's proportions, which makes them furious believing it was a lie that Twilight would not survive her friends.

But being honest, I doubt that much. I see that it is normal for the long time that has passed more or less 20 years, I see that it is normal for Twilight to have reached the same proportions as Celestia, I think Cadence would also have the same size, I think that is a size is part of the growth of an alicorn, apart, we must take into account that Celestia and Luna existed BEFORE Equestria existed, if they were already immortal for some other reason, I doubt that Twilight is also just because now Equestria rules, in addition, in the castle we saw a showcase where we see Flurry Heart becoming the new ruler of the crystal empire, so that was not a job that his mother would do for eternity.



We have also seen the other alicorn princesses grow old normally, when we met Cadence, we saw her as a teenager when she was taking care of Twilight and currently as an adult, and we saw an adult Flurry Heart in the showcase, seeing that she also ages normal if like Cake twins, she passed into adulthood.



If Twilight really was immortal, poisoning having grown up should be the same alicorn of the last 6 seasons, because it is assumed that part of immortality is never getting old, or taking several decades or centuries to change a little, making one Be young at the age when a mortal is old.