What effects of a magical strength increase that occurs during, and only during, the teenage years, have on human society and infrastructure?
Like perhaps all strength is multiplied by 2, but all muscles and stuff act as if the teenager was only at normal, for us, strength.
But that's only to allow the strength increase to be used. Any secondary effects, like being hit by another teenager, would have results like the actual doubled strength.
Also, strength regulation is properly adjusted, again, via magic.
The teenage-years are defined as every year of age that, in English, has the suffix '-teen' in it, and strength arrives and leaves at midnight of the start, and end of the birthdays that mark the beginning and end of such years.
... That might be slightly, slightly useful for age identification?