Age 4-6 teach child to swim and develop swimming form, etc.
Ages 7-9 develop as a swimmer (swim 5 days a week 1 hour per day). Have child participate in short fast races and maybe a 5k race here or there. Limit running to 10 miles per week.
Ages 10-13 develop further as a swimmer (swim 6 days a week 2 hours a day). Begin running more progressing up to 40 miles per week at age 13. Participate in some 5k's.
Ages 14-16 maintain swimming, Build running mileage up to 90 miles per week, start to train seriously as a runner, and compete in two seasons per year of running (XC and track). Should focus on shorter faster events such as 800m and 1600m (and run 4x400s as well)
Age 17-22
Drop swimming down to an hour a day. Build mileage to 120 miles per week. Training will become more serious at this time. Continue to work on speed, but start progressing up distance in both races and practices.(Remember most world records are hit in this time period).
Ages 23-retirement
See training that Renato talks about in previous threads. Running is a full-time professional career.