I believe the proportion of young people acting younger than their age has risen, yes, though the majority are still acting their age.
I did notice our crop of year 7 last year was particularly immature and I do think that they suffered from not having appropriate access to their last two years of primary school where a lot of the maturing happens.
But, as ever, so much is down to societal standards and parenting.
If I compare the general maturity of English children to that of the country I grew up in, there is a marked difference in what is being expected of children.
It was normal to walk to school by ourselves age 6+, aged just 9/10 I had to take two inner-city trains and a tram to get to where I needed to be, daily, by myself, and no one batted an eyelid.
Where I live now, children are forbidden from walking themselves to school until they are at least 10, even if home is less than 10min walk away.
We were responsible for buying and maintaining all of our school equipment, even text books, and had to pack them ready for whatever lessons we had that day. Failure to bring in homework resulted in a fail grade - too many of those and we'd be made to repeat that school year. Here in England, where I teach, many students don't even bring a pen to school and it is somehow on the teacher to provide students with basic materials, because if they don't, it becomes the teacher's fault down the line.
In my country of origin, no one would think twice about telling someone else's children off if they were doing something wrong - I remember getting shouted at once for jaywalking (and pulled up by a cop a few years later for attempting to do the same), a tram driver once pulled me into a tram full of people to have a go at me for not looking before I crossed the tracks. Believe me, when you had a bollocking like that in front of a crowd of strangers, you only ever do that once.
In short, as a society in England (I can't speak for the rest of the UK) we mollycoddle children far too much and that does result in more immature children overall.
My children are thought of as being more mature than many of their peers, but I hold them to the same standards I was held to as a child - including set bedtimes, chores, freedoms with responsibilities and I put in the hard work in the early years to make them independent early on, which does result in better maturity.
The eldest is also encouraged to work - they have helped out in the school kitchen in Y6, then became a school librarian and a language tutor to younger students at secondary level and we are now looking at a small paid job as they are soon approaching 16. All of which helps them massively in developing skills and have a much more mature attitude.