In performance sports constant fitness regimes and a lifestyle that is focused on performance are imperative.
There is no fast way to sporting results for most athletes, not all are fast starters and many reap the results of their training and clean living efforts later on in life. Remember you can establish a swimming record till very late in life, age-groups never stop.
Learning to deal with rejections and throwbacks is almost as important for a young swimmer as being able to win. Most swimmers probably lose more races than winning them.
It is however very important to attend competitions on a monthly basis to stay tuned.
In swimming as a sport, peaking at 18+ is probably more convenient than earlier because it fits in with the schooling regime that we all have to follow here in the UK.
Performance swimming means being constantly on the swim, on a daily basis. You gotta love swimming a lot to be able to do it.
Once you get selected for national teams, you get a whole host of wonderful training opportunities through podium funding. Prior to that all athletes can apply for GLL funding. But as said previously there are also many practical ways to improve fitness.