Well - this all depends! I am now 9 stone 10 and my skin is just fine HOWEVER my tummy is not great... this is nothing to do with the diet/ how I've lost weight/how quickly and everything to do with having two big babies and stacking on 6 stone with my first pregnancy. So... you need to consider a few factors. How old are you? how much do you need/want to lose? If you are 20 stone, for example, then yes, expect to have some loose skin at the end of the journey. If you are 13 stone then you will probably be OK.
I am 40 and I suppose i have 'good' skin. And anyway... compared to being overweight and feeling down about that, the loose skin thing is really nothing to worry about. I also think it is a way to 'self sabotage ' yourself. ' Oh, what is the point of losing weight... I'll be left with sagging skin ..might as well not bother.. '
Concentrate on the job in hand - losing the weight. THEN fret about what you're dealing with come target. Think positive , affirming things and you'll be just fine.