I think this could have been handled a lot better than it was, tasering her, even if he mistook her for an adult seems excessive.
...but as for the age mistake...I really can't say "he should have known". All the hormones in our food and water anymore aren't helping, puberty used to begin around 12, now they consider 9 or 8 even in the "normal" range.
Son has a friend that looks every inch of 16 or older, (she's a size D)... and way taller than I am (which isn't too difficult I guess
)...she'll be 12 in a few weeks.
Precocious Puberty (in that case)...has some girls starting as young as 5 or 6.
Not saying this is the case here, just saying mistaking her for being older...I would say "possible".