A child, teen, or adult ballet student will need a different approach. Some may need to get in shape for ballet for a few months.
Unless there is a medical condition, there is no reason not to go back to classical dancing.
For example, perhaps you took ballet classes from the age of five to fifteen.
Then for some reason you decided to go to college. You pursue academics for a couple of years, and realize you really miss dancing. You were considered a talented student formerly, but now you worry about what it would take to get back to an advanced level.
Early in the year, look into summer intensives. The more established summer intensives will ask you to audition, and may have already held their auditions.
Sometimes a smaller more local studio is the best, as you may get more attention in those classes than in a major ballet school.
After intensive study, you and your teachers will have a good idea of whether you can make the progress you want.
But you don't have to wait....if you can, right now, take a weekly ballet class, starting in a less advanced class than you had been doing so you can have "the luxury" of concentrating on basic ballet technique. The fancy combinations of an advanced class would be a distraction.
In some cases a dedicated student gets accepted into a full time training ballet school, and "something happens". After recovery from injury or some other situation, has taken place, an ambitious teen can be tortured by not knowing how the previous scenario would have played out - and wonders if she/he should try to get back to it.
I say if you can, audition again. Some of the training schools have a special course for older students who are talented enough to make quick progress, or go into a teacher training program that dovetails with a degree program.
Sometimes a teacher is approached by a student who quit ballet at the very old age of ten, after five years of classes. Now she/he wants to get back into class, and isn't sure how to catch up.
Many studios like to keep classes geared to an age range, but I would put this student into a very basic technique class once a week for a while. She/he could also take a class closer to the previous level studied. That way boredom won't take over.
Doctors I have asked say that the best cardio/endurance training for dancers is swimming. There is no impact on the joints at all, as there is with jogging or running.
If you want some ballet barre exercises explained to practice effectively, use The Perfect Pointe Book to get back into pointe shoes.