Why don't people understand that it's relatively normal for young athletes to succeed in swimming. It's not possible in other sports but for some reason it is in swimming - just accept it. With that said - massive under performance to not even get a silver.
I wonder, with all these young swimmers coming out of nowhere and performing incredibly (Shiwen, Franklin and ledecky), it's more sensible to assume that 15/16/17 year olds can do this rather than stocking up on PED. It's unbelievable but it doesn't seem to be impossible.
As a few others have mentioned it's purely a mental thing.
People who've never done competitive swimming training don't realise what a mentally draining sport it is. The training is incredibly boring compared to most other sports and coupled with the early starts it takes a lot of willpower to keep yourself motivated. Once that starts to wane even a little you're finished. Don't underestimate how hard it is to get up on a freezing cold winter morning knowing you're about to jump in some water for two hours.
Combine that with the fact that you have to start at a really young age to get to Olympic level and it's not surprising many get burnt out by their early 20'
I did it for about 13 years until my late teens (much lower level obviously) and I'd put myself through 100 back-to-back footy preseasons before I'd ever get back in the pool.
Although Female swimmers should mature physically quicker then guys, same can be said for sports like Gymnastics and Tennis.