Nedul--fact is that the job requires a uniform and a uniform that is worn properly. If you can't follow that, rather basic, requirement then your ability to follow other requirements is suspect.
I don't care if you are 21 or 61, the image you portray to the traveling public is important which is why airlines push as hard as they do regarding their uniform standard.
Finally, if the employer requires it, whether you like it or not, you WILL wear the uniform as stated. If not you won't have a job.
If it is, in fact, a millennial issue then they will learn soon enough - the hard way. But I don't think it is actually a millennial issue. Rather it is an issue with a few fools who think they can make their own rules. An issue with 20-somethings for generations and not specific to this generation.