If 青春無敵 just means no noticeable wrinkles on the face, it is cheap and a few plactic surgeries can do wonders to many.
If it means have bold dreams about life, then it can be associated to people at any age,not just the people in their 20's.
As long as the 90後 you are talking about are not completely consumed by money and the willingness to get rich at all cost, it is good for them that they can pull off a late night without showing signs of aging the next day.Just be aware, with the stationary lifestyle that are common to most people in China, it is interesting to see how long 青春 can last. My experience, it is shorted than elsewhere.
