Again, that would make sense if it applied equally for all new drivers - but if you're over 25 then there is no such restriction, even if you got your licence a day before. You have zero experience behind the wheel but you're fine to drive in a car full of people, but someone who has been driving for 7 years but is one day short of 25 can't do it - who is the more experienced driver there?
So yeah, it's all about "not being mature enough".
So yeah, it's all about "not being mature enough".