It is possible to lose weight at any age, but to be honest, having got so fat you need to diet/ lose a large amount of weight by the age of 22 means you are probably far more likely to pile it back on again.
I think once you have got to the stage of having to diet, you have already lost part of the battle. That's not meant to be offensive to anyone, and I'm not implying anyone is weak or whatever by having got to this place. However, excluding underlying medical conditions, if you have put on lots of weight it is probably due to overestimating the amount of food you need to eat, eating the wrong things, or having a poor relationship with food.
Once you have got into those habbits it must be much harder to break them than never having got into those habbits at all. I imagine that is why a lot of diets don't work for people- they can't change to a healthy lifestyle, or have never properly learnt what one is. There is a lot more education about this (imo) now than there was in the past.
It's not about weight as such, anyway. It's about being healthy and fit for your natural body shape. In my teens I was obsessed with being really skinny, although my natural body shape is more hourglass/curvy. I weighed probably about 1/2-1 stone less than I do now, and I was fainting and feeling light headed on a semi-regular basis. That's no more healthy than being fat.
Now I eat more sensibly and try to do regular exercise to stay fit. It's not rocket science, but if people never learn what eating sensibly is, or what a sensible amount of exercise is, then this is very hard for them to achieve. "Diets" that involve cutting out whole food groups, meal replacement, and crazy detoxes don't help with this at all, really.