It is very acceptable for younger people to have lower standards of living in economic terms than their parents. Young people are just starting out and they are not established. They are not going to be able to afford the things that their paretns can because they are just beginning.
Yes, it is acceptable for young people to have lower standards of living than their parents. A standard of living should be earned. Many parents worked very hard to earn their high standard of living. Young people are not entitled to a certain standard of living until they have also earned it.
Yes, it is acceptable for young people to have lower standards of living than their parents. Unless they come from a highly privileged family, or somehow made connections in their youth that have allowed them to leapfrog the first few rungs of the career ladder, it is natural for young people to have lower standards of living. Most young people are yet to gain the skills or experience that their parents have and as a result have not yet reaped the rewards that come from utilising those skills. Ideally, everyone would have excellent living standards their whole lives, but for most young people a lower standard of living is just a temporary stepping stone before moving on to bigger and better things.
The statement implies that relative to when their parents were the same age as them, they are worser off. 20 year olds today are worser off then 20 years 30 years ago. It isn't comparing their current position. Of course it is acceptable to not be as well off as their parents currently since their parents are older. However, it is not acceptable for them to be worse off at 20 then when their parents are 20. If this trend continues each future generation will have a worse life then the generation before them. Our parents should have worked to insure that their kids had better lives, but they failed.
Young people should have a better standard of living than their parents; progress should move forward, as should the standard of living. The function of society as a greater construct is firstly to ensure health and happiness among its people, and second to progress forward in order to improve that health and happiness. When young people are born into a world with a lower standard of living than the generations that came before them, it marks a failure of society in the duties that it has been tasked with.