2008 got to a point whereby only 3 housemates were over the age of 22 about 2/3rds of the way through the season and that did feel a little odd. Like a bunch of naughty school kids and their 2 'care givers' that none of them listened to. Plus Alice who just ran on the treadmill all day I guess!
I'd say maybe 2/3rds of the cast under 40 and 1/3 over 40 would be a decent mix. These days we tend to get less contestants under 25 in most reality shows other than maybe Love Island so I think this season definitely won't be as young as some of the Ten seasons were age wise at any rate.
At this point I'm worried if we'll get the house done in time, never mind the time slot. those progress stories have worried. Are ten trying to compete with the block in real time?