unfortunately not its not fixed which is what we thought.
We applied when my son was 17 and at college our application 136 (now 175) was submitted in March our son turned 18 in May (this is all last year) we got a CO in August, meds in Sptember meds cleared in December.
In October he decided to leave college (in his wisdom)
We are now battling to prove dependency

which looks like its a long shot for us as he is working but not earning enough to be independant but its proving that.
They dont ask for prove until you are at decision stage so we went through all of this and he could still fail
I have heard a small rumour that this may be dependant on the visa you are applying for but not sure of what types that is, sorry.
My son doesnt want to come either but hoping to change is mind also.
Try and gather as much prove he is dependant on you as possible any bills you pay for him is a good one possibly.
Good luck with this.