Wigs
boys start to lost flexibility from the age about 10 to 14 ( girls have better flexibility ). therefore flexibility training ( include soccer specific flexibility training ) is very much needed for them. and also to all age group. it's very IMPORTANCE!! its enhance other physical fitness and also improve soccer performance. however, flexibility training is the most neglected physical component in soccer training. you should maintain flexibility training in all your training session unless you have reason not to do so.
about strength, under 12 can use general exercise to build their basic strength. avoid high impact activities to avoid injury.
13 to 16. build strength using their own body weight. Phuabh