Ah, I meant to add that detail and forgot until you mentioned it. If I'm rapping these off in only a few hours each I miss things like cutie marks and such at the end. Well, this time I'll leave it as is.
If she could do that, then Celestia could too, and yet she doesn't. Why? Because that would mess with the natural order of things. Ponies grow old and die. It's a natural part of life, and Celestia knows nopony, not even herself, has any business interfering with nature's course for any reason. Though it would be very painful for her, she would forbid Twilight from making her friends immortal. Because her friends have family too, and they would outlive their family. How do you think that would make them feel, watching their loved ones grow old and die while they stay young forever? Such is the curse of immortality, and alicorns alone must bear that heavy burden. This is one reason why I vehemently protest alicorn Twilight.
I know this post was made a while ago but. It's been stated that Twilight will not outlive her friends, so wither she uses an age spell they become immortal or far mroe likely Twilight is not anymore long lived/immortal than she was before. Personally I think being immortal would be great and that natural immortals would be biologically designed to cope with living around mortals.