As an under 25 year old you lack experience. Youth unemployment (which is defined as under 25) is particularly high. So the difference in wage makes you more competitive and attractive in the job market. This is coupled by the huge push for apprenticeships which are only available up to 25 years old. When you apply to university, you are assessed based on your parents income up to the age of 25 if you are dependant on them.
So the whole set up is built around the fact that under 25 you are expected to be in education or training of some description. Which does make sense on paper and has been consistent with previous government policy and classification for years - its not a new Tory invention for this budget.
Of course there are real problems with this. Not just for the under 25s. Would you at age 26 want to now start a family? You are less likely to have savings built up as employers who would previously paid more for a 21 year old may simply reduce their pay structures for those who are younger. And of course it completely neglects to acknowledge that there are loads of people who are married and have kids by this age. It rather infantilises under 25 year olds, rather than encourage them to take on responsibility.
There is an area where age discrimination under law might be an issue though - if someone has the same responsibilities and experience in a particular field as a college but is paid substantially less then an employer may in fact be breaking the law. It was less of an issue when the lower wage cut off was 21 as the amount of experience you can have accumulated simply isn't as much, but with a bigger range this might be more of an issue. The trouble is not many people will be aware of it and fewer still will want to or have the funds to pursue it legally. Age discrimination cases to date have generally centred around older people and have been supported by charities with an interest in the old. However there are fewer large charities dedicated to youths and they don't have the same power as the likes of Age UK. A MP under could not legally be paid less than any other MP on the grounds of age for example.
Not only that but I graduated before my 21st birthday. There will be lots of graduates in a similar position. Being a summer baby rather than an Autumn baby has yet another disadvantage. My point being that it will lead to graduate jobs being replaced by lower paid as 'training or apprenticeship' posts rather than graduate jobs that have more status and pay. Especially since large businesses are being forced invest more in these types of post. What no one has thought about is this means that less people will be repaying student loans straight away and since they are then written off after 30 years under the current rules, it means that there is going to be a bigger shortfall long term which is already being criticised as not bringing in enough money (so don't be surprised if there are further changes to loans system).
Under 25s are far more likely to be single and therefore more able and willing to live in shared accommodation even if they don't live with parents.
However the cost of accommodation has not been properly addressed for ANY group and personally I do think in many respects affordability of housing is a separate issue rather than one restricted to the young.